1. UNIT V DC-DC CONVERTERS

2. Topics to be covered
IGBT - Structure and operation, Static and dynamic switching characteristics, Performance parameters, gate drive requirements, Input and output characteristics, Applications.
Performance Parameters of DC–DC Converters
Principle of Step-Down, Step-Up Operation
Frequency Limiting Parameters
Converter Classification
Switching-Mode Regulators
Buck, Boost, Buck- Boost,Cuk converters
Limitations of Single-Stage Conversion
Comparison of Regulators

3. IGBT- Insulated Gate Bipolar Transistor
The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with high input impedance and large bipolar current-carrying capability.
The IGBT is suitable for many applications in power electronics, especially in Pulse Width Modulated (PWM) servo and three-phase drives requiring high dynamic range control and low noise.
It also can be used in Uninterruptible Power Supplies (UPS), Switched-Mode Power Supplies (SMPS), and other power circuits requiring high switch repetition rates.
IGBT improves dynamic performance and efficiency and reduced the level of audible noise.
The IGBT is suitable for scaling up the blocking voltage capability.

4. Basic Structure
The basic schematic of a typical N-channel IGBT based upon the DMOS process
This is one of several structures possible for this device

5. Some IGBTs, manufactured without the N+ buffer layer, are called non-punch through (NPT) IGBTs whereas those with this layer are called punch-through (PT) IGBTs.
The presence of this buffer layer can significantly improve the performance of the device if the doping level and thickness of this layer are chosen appropriately.
Despite physical similarities, the operation of an IGBT is closer to that of a power BJT than a power MOSFET.
Equivalent circuit model of an IGBT
IGBT Circuit Symbol

6. Structure (a) NPT-IGBT and (b) PT-IGBT

7. Operation Modes Forward-Blocking and Conduction Modes
When a positive voltage is applied across the collector-to-emitter terminal with gate shorted to emitter , the device enters into forward blocking mode with junctions J1 and J3 are forward-biased and junction J2 is reverse-biased.
A depletion layer extends on both-sides of junction J2 partly into P-base and N-drift region.
An IGBT in the forward-blocking state can be transferred to the forward conducting state by removing the gate-emitter shorting and applying a positive voltage of sufficient level to invert the Si below gate in the P base region.
This forms a conducting channel which connects the N+ emitter to the N- drift region.
Through this channel, electrons are transported from the N+ emitter to the N- drift.
This flow of electrons into the N- drift lowers the potential of the N- drift region whereby the P+ collector/ N- drift becomes forward-biased.
Under this forward-biased condition, a high density of minority carrier holes is injected into the N- drift from the P+ collector.

8. When the injected carrier concentration is very much larger the background concentration, a condition defined as a plasma of holes builds up in the N- -drift region.
This plasma of holes attracts electrons from the emitter contact to maintain local charge neutrality.
In this manner, approximately equal excess concentrations of holes and electrons are gathered in the N- drift region.
This excess electron and hole concentrations drastically enhance the conductivity of N- drift region. This mechanism in rise in conductivity is referred to as the conductivity modulation of the N- drift region.
Reverse-Blocking Mode
When a negative voltage is applied across the collector-to-emitter terminal, the junction J1 becomes reverse-biased and its depletion layer extends into the N- drift region.
The break down voltage during the reverse-blocking is determined by an open-base BJT formed by the P+ collector/ N- /drift/P-base regions.
The device is prone to punch-through if the N- -drift region is very lightly-doped.
The desired reverse voltage capability can be obtained by optimizing the resistivity and thickness of the N- -drift region.

9. The width of the N- drift region that determines the reverse voltage capability and the forward voltage drop which increases with increasing width can be determined by
Reverse blocking IGBT is rare and in most applications, an anti-parallel diode (FRED) is used.

10. Output Characteristics
Output I-V characteristics of an NPT-IGBT [IXSH 30N60B2D1]
When the gate emitter voltage is below the threshold voltage only a very small leakage current flows though the device while the collector – emitter voltage almost equals the supply voltage.
The device, under this condition is said to be operating in the cut off region.

11. As the gate emitter voltage increases beyond the threshold voltage the IGBT enters into the active region of operation.
In this mode, the collector current ic is determined by the transfer characteristics of the device.
This characteristic is qualitatively similar to that of a power MOSFET and is reasonably linear over most of the collector current range. The ratio of ic to (VgE – VgE(th)) is called the forward transconductance (gfs) of the device and is an important parameter in the gate drive circuit design.
As the gate emitter voltage is increased further ic also increases and for a given load resistance (RL) VCE decreases.
At one point VCE becomes less than VgE – VgE(th). Under this condition the driving MOSFET part of the IGBT enters into the ohmic region and drives the output p-n-p transistor to saturation.
Under this condition the device is said to be in the saturation mode. In the saturation mode the voltage drop across the IGBT remains almost constant reducing only slightly with increasing VgE.

12. Transfer Characteristics
The transfer characteristic is defined as the variation of ICE with VGE values at different temperatures, namely, 25 oC, 125 oC and -40 oC.
The gradient of transfer characteristic at a given temperature is a measure of the transconductance (gfs) of the device at that temperature.

13. A large gfs is desirable to obtain a high current handling capability with low gate drive voltage.
The channel and gate structures dictate the gfs value.
Both gfs and RDS(on) (onresistance of IGBT) are controlled by the channel length which is determined by the difference in diffusion depths of the P base and N+ emitter.
The point of intersection of the tangent to the transfer characteristic determines the threshold voltage (VGE(th)) of the device.
Transconductance Characteristics of an IGBT

14. Switching characteristics of IGBT
Switching characteristics of the IGBT will be analyzed with respect to the clamped inductive switching circuit
Inductive switching circuit using an IGBT
Equivalent circuit of the IGBT

15. The switching characteristics of an IGBT are very much similar to that of a Power MOSFET. The major difference from Power MOSFET is that it has a tailing collector current due to the stored charge in the N- - drift region.
The tail current increases the turnoff loss and requires an increase in the dead time between the conduction of two devices in a half-bridge circuit.
IXYS IGBTs are tested with a gate voltage switched from +15V to 0V. To reduce switching losses, it is recommended to switch off the gate with a negative voltage (-15V).

16. Switching waveforms of an IGBT.
The switching waveforms of an IGBT is, in many respects, similar to that of a Power MOSFET. This is expected, since the input stage of an IGBT is a MOSFET.
Also in a modern IGBT a major portion of the total device current flows through the MOSFET.
Therefore, the switching voltage and current waveforms exhibit a strong similarity with those of a MOSFET.
However, the output p-n-p transistor does have a significant effect on the switching characteristics of the device, particularly during turn off.
Another important difference is in the gate drive requirement. To avoid dynamic latch up, the gate emitter voltage of an IGBT is maintained at a negative value when the device is off.

17. The gate drive circuit of an IGBT should ensure fast and reliable switching of the device. In particular, it should:
Apply maximum permissible VgE during ON period.
Apply a negative voltage during off period.
Control dIc/dt during turn ON and turn off to avoid excessive Electro magnetic interference (EMI).
Control dVcc/dt during switching to avoid IGBT latch up.
Minimize switching loss
Provide protection against short circuit fault.

18. IGBT gate drive circuit
Equivalent circuit of the gate drive during turn on and turn off.

19. Performance parameters
Maximum collector-emitter voltage (VCES): This rating should not be exceeded even on instantaneous basis in order to prevent avalanche break down of the drain-body p-n junction. This is specified at a given negative gate emitter voltage or a specified resistance connected between the gate and the emitter.
Maximum continuous collector current (IC): This is the maximum current the IGBT can handle on a continuous basis during ON condition. It is specified at a given case temperature with derating curves provided for other case temperatures.
Maximum pulsed collector current (ICM): This is the maximum collector current that can flow for a specified pulse duration. This current is limited by specifying a maximum gate-emitter voltage.
Maximum gate-emitter voltage (VgES): This is the maximum allowable magnitude of the gateemitter voltage (of both positive and negative polarity) in order to
• Prevent break down of the gate oxide insulation. • Restrict collector current to ICM.
Collector leakage current (ICES): This is the leakage collector current during off state of the device at a given junction temperature. This is usually specified at VgE = 0V and vCE = VCES.

20. Gate-emitter leakage current (IGES): Usually specified at vCE = 0V & vgE = vgES.
Collector emitter saturation voltage (VCE(sat)): This is specified at a given junction temperature, gate-emitter voltage and collector current. For more detailed data the output characteristics of the device for different vgE and expanded near the saturation zone is also provided.
Gate-emitter threshold voltage (vgE(th)): It is specified at a low collector emitter voltage and collector current.
Forward Transconductance (gfs): This is again specified at a low value of vCE. For more detailed data the transfer characteristics of the device (ic vs vgE) is also provided.
Input, output and transfer capacitances (Cies, Coes & Cres): These are, gate-emitter, collectoremitter and gate-drain capacitances of the device respectively, specified at a given collectoremitter voltage. Variation of these parameters as functions of vCE are also supplied.
Switching times (td(ON) tri, tfv, trv, tfi): These times are specified for inductive load switching as functions of gate charging resistance and collector current. In addition turn on and turn off energy losses per switching operation are also specified.
Maximum total power dissipation (Ptmax): This is the maximum allowable power lass in the device (both switching and conduction) on a continuous basis at a given case temperature. Derating curve at other temperatures are also specified.

21. Safe operating area of an IGBT (a) FBSOA; (b) RBSOA.
The IGBT has robust SOA(Safe operating area) both during turn on and turn off.
On the left side it is restricted by the forward voltage drop characteristics.
Up to maximum continuous collector current this voltage remains reasonably constant at a low value.
However, at ICM this voltage starts increasing as the IGBT starts entering active region. On the top the FBSOA is restricted by ICM.
Safe operating area of an IGBT (a) FBSOA; (b) RBSOA.

22. Applications and advantages
The IGBT combines the simple gate-drive characteristics of MOSFETs with the high-current and low- saturation-voltage capability of bipolar transistors.
The IGBT combines an isolated-gate FET for the control input and a bipolar power transistor as a switch in a single device.
The IGBT is used in medium- to high-power applications like switched-mode power supplies, traction motor control and induction heating.
Large IGBT modules typically consist of many devices in parallel and can have very high current-handling capabilities in the order of hundreds of amperes with blocking voltages of 6500 V.
These IGBTs can control loads of hundreds of kilowatts.

23. Performance Parameters of DC–DC Converters
High Electrical Performance
Accurate Output Voltage
Excellent Load and Line Regulation
Very Small Load Transient Overshoot and Fast Transient Response
Very Low Turn on Overshoot
High Efficiency
Ultra Low Noise
Low Reflected Input Ripple Current
Loop Stability

24. Fault Protection
Lower Short Circuit Current to Full Scale Current Ratio
Thermal Shutdown
Over-Voltage Protection
Under-Voltage Lockout
High Reliability
Useful Functions
Remote On/Off Control
Output Voltage Trim
Cold Start at –40oc
Isolation between Input and output

25. DC-DC Switch-Mode Converters
Applications:
Regulated switch mode dc power supplies
dc motor drives
dc-dc Converters:
Step-down (buck) converter
Step-up (boost) converter
Step-down/step-up (buck-boost) converter
Cuk converter
Full-bridge converter

26. Functional Block Diagram of DC-DC Converter System
Controlled dc output at a desired voltage level
Unregulated dc voltage obtained by rectifying the line voltage, and therefore will fluctuate with line voltage magnitude

27. Control of DC-DC Converters
In a dc-dc converter:
Average output dc voltage must be controlled to equal a desired level.
Utilizes one or more switches to transform dc from one level to another.
The average output voltage is controlled by controlling the switch on and off durations (ton and toff).
Let’s consider the following switch-mode dc-dc converter:
Average output dc voltage Vo depends on ton and toff.
Switching is done at a constant frequency with switching time period Ts.
This method is called pulse-width modulation (PWM) in which the duty ratio D is varied to control Vo, where D=ton/Ts

28. Control of DC-DC Converters (cont’d)
The switch control signal, which controls the on and off states of the switch, is generated by comparing a signal level control voltage vcontrol with a repetitive waveform.
The switching frequency is the frequency of the sawtooth waveform with a constant peak.
The duty ratio D can be expressed as

29. Step-Down (Buck) Converter
converts dc from one level to another
the average output voltage is controlled by the ON-OFF switch
pulse-width modulation (PWM) switching is employed
lower average output voltage than the dc input voltage Vd depending on the duty ratio, D
D=ton/Ts
Average output:
Applications:
regulated switch mode dc power supplies
dc motor drives
low-pass filter: to reduce output voltage fluctuations
diode is reversed biased during ON period, input provides energy to the load and to the inductor
energy is transferred to the load from the inductor during switch OFF period
in the steady-state, average inductor voltage is zero
in the steady-state, average capacitor current is zero

30. Step-Down (Buck) Converter: Continuous current conduction mode
Inductor current iL flows continuously
Average inductor voltage over a time period must be zero
Assuming a lossless circuit
Buck converter is like a dc transformer where the turns ratio can be controlled
electronically in a range of 0-1 by controlling D of the switch

31. Example…..
For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio
and “on” time of the switch.

32. Solution….
D = V0 /Vd = 5/40 = = 12.5%
Ts = 1/fs = 0.25 ms = 250 ms
Ton = DTs = ms
Toff = Ts – ton = ms
When the switch is “on”: VL = Vd - V0 = 35 V
When the switch is “off”: VL = -V0 = - 5 V
I0 = IL = V0 / R = 5 A
Id = D I0 = A

33. Step-Up (Boost) Converter
Output voltage always higher than the input voltage
When the switch is ON:
diode is reversed biased
output circuit is thus isolated
inductor is charged
When the switch is OFF:
the output stage received energy from the inductor as well as from the input
Filter capacitor is very large to ensure constant output voltage
Applications:
regulated switch mode power supplies
Regenerative braking of dc motors

34. Step-Up (Boost) Converter: Continuous current conduction mode
Inductor current iL flows continuously
Average inductor voltage over a time period must be zero
Dividing both side by Ts
Assuming a lossless circuit

35. Step-Up (Boost) Converter: Effect of parasitic elements
Parasitic elements are due to the losses associated with the inductor, capacitor, switch and diode
Figure shows the effect of the parasitic elements on the voltage transfer ratio
Unlike ideal characteristics, in practice, Vo /Vd declines as duty ratio approaches unity

36. Step-Down/Step-Up (Buck-Boost) Converter
This converter can be obtained by the cascade connection of two converters: the step-down converter and the step-up converter
The output voltage can be higher or lower than the input voltage
Used in regulated dc power supplies where a negative polarity output may be desired with respect to the common terminal of the input voltage
The output to input voltage conversion ratio
This allows V0 to be higher or lower than Vd
When the switch is ON:
diode is reversed biased
output circuit is thus isolated
inductor is charged
When the switch is OFF:
the output stage received energy from the inductor

37. Buck-Boost Converter: Continuous current conduction mode
Inductor current iL flows continuously
Average inductor voltage over a time period must be zero
Assuming a lossless circuit
Depending on the duty ratio, the output voltage can be either higher or lower than the input

38. Buck-Boost Converter: Effect of parasitic elements
Parasitic elements are due to the losses associated with the inductor, capacitor, switch and diode
Parasitic elements have significant impact on the voltage transfer ratio

39. Cuk DC-DC Converter Named after its inventor
The output voltage can be higher or lower than the input voltage
Provides a negative polarity output voltage with respect to the common terminal of the input voltage
C1 acts as the primary means of storing and transferring energy from the input to the output
In the steady-state, average inductor voltages, VL1 and VL2 are zero, therefore,
VC1 = Vd + V0

40. Cuk DC-DC Converter When the switch is OFF:
- iL1 and iL2 flow through the diode
- C1 is charged through the diode by energy from both the input and L1
- energy stored in L2 feeds the output
When the switch is ON:
- Vc1 reverse biases the diode
- iL1 and iL2 flow through the switch
- since Vc1>V0, C1 discharges through the switch, transferring energy to the output and L2
- Therefore, iL2 increases
- Input feeds energy to L1 causing iL1 to increase

41. Steady-state current and voltage equations…………..Cuk
Vc1 is constant and average voltages across L1 and L2 over a time period must be zero
Equating the above two equations,

42. Comparison of Converters
Buck converter: step-down, has one switch, simple, high efficiency greater than 90%, provides one polarity output voltage and unidirectional output current
Boost converter: step-down, has one switch, simple, high efficiency, provides one polarity output voltage and unidirectional output current, requires a larger filter capacitor and a larger inductor than those of a buck converter
Buck-boost converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal
Cuk converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal, additional capacitor and inductor needed
Full-bridge converter: four-quadrant operation, has multiple switches, can be used in regenerative braking

43. Conclusions
In many industrial applications, it is required to convert fixed dc voltage into variable dc voltage
Various types of dc-to-dc converters
Operation of dc-to-dc converters
The step-down, step-up, buck-boost and Cuk converters are only capable of transferring energy only in one direction
A full-bridge converter is capable of a bidirectional power flow
Like ac transformers, dc converters can be used to step-up or step-down a dc voltage source
Applications: electric automobiles, trolley cars, marine hoists, mine haulers, etc.
Also used in regenerative braking of dc motors to return energy back into the supply –energy savings for transportation systems with frequent stops

44. Comparison of Regulators
A DC/DC converter that stabilizes the voltage is often referred to as a voltage regulator.
Two types of regulators exist, classified by a conversion method: linear or switching.
Linear Regulator
A linear regulator is one where a linear component (such as a resistive load) is used to regulate the output.
It is also sometimes called a series regulator because the control elements are arranged in series between the input and output.

45. Switching Regulator
A switching regulator is a voltage regulator that uses a switching element to transform the incoming power supply into a pulsed voltage, which is then smoothed using capacitors, inductors, and other elements.
Power is supplied from the input to the output by turning ON a switch (MOSFET) until the desired voltage is reached.
Once the output voltage reaches the predetermined value the switch element is turned OFF and no input power is consumed.
Repeating this operation at high speeds makes it possible to supply voltage efficiently and with less heat generation.