Presented by: Sanjay Pithadia SEM – Industrial Systems, Medical Sector Designing High-Voltage, Programmable Power Supply for driving High-current Pulsed loads Part - 3 Presented by: Sanjay Pithadia SEM – Industrial Systems, Medical Sector Hello  everyone and Welcome to Part three of the training series on Designing High-Voltage Programmable Power Supply for driving high-current pulsed loads.

Challenge to Solve Scaling Voltage and Current using standard voltage regulators and MOSFETs In  this video, we will go through tricks of scaling voltage and current using standard voltage regulators and MOSFETs.

Voltage Scaling – to support Higher Input Voltage 150𝑉 160𝑉 150−100 2 +100=125V 125−0.7=124.3𝑉 125−0.7=124.3𝑉 160−90 2 +90=125V The regulator’s operating input voltage range decides VIN and VG values Truly scalable Can be increased by adding more BJTs 124.3−100=24.3𝑉 124.3−90=34.3𝑉 To Input of Voltage Regulator VIN = Input Voltage VG = Virtual Ground Voltage 100𝑉 90𝑉 Let  us assume that we want our voltage regulator to support higher input voltage without crossing its absolute maximum ratings and damaging the regulator. Here  is a simple circuit using three resistors and a bipolar junction transistor or BJT. VIN  is the input voltage and VG is the virtual ground voltage. Let’s look at the operation of this circuit. The two 10 kilo ohm resistors R1 and R2 decide the voltage at the base  of BJT T1 – marked as VB. The Voltage on Emitter of BJT  is marked as VE. A sense resistor of 47 ohm R3 will be used for  current scaling in upcoming slides. The  input to the voltage regulator is voltage difference between VE and VG . Let’s say we set the input voltage at 150  volts and virtual ground is set at 100  volts. Since values of R1 and R2 are equal, the voltage on base of transistor T1 equals  125 volts. Assuming base-to-emitter voltage VBE of T1 equals 0.7V, The voltage VE  is equal to 124.3 volts. The difference between VE and VG is equal to 24 .3 volts which is the actual input to the regulator. Let’s  take similar example with input voltage at 160 volts and virtual ground is set at 90  volts. In this case, VB equals 125V  and VE equals 124 .3V, the input voltage to regulator is 34 .3 volts. The circuit is flexible enough to make sure the recommended input voltage ratings are not crossed, by setting the input voltage and virtual ground. TI’s  TPS7A47 is 36V-positive voltage regulator. While using it with such voltage scaling circuit, the designer needs to make sure the input voltage is less than 35 volts. In  summary, the voltage regulator’s recommended operating input voltage decides the VIN and VG values of this scaling circuit. More  number of bipolar junction transistors can be added to scale the voltages further – so this scaling circuit is truly scalable. One important thing to note that the transistor should have voltage rating higher than VIN and VG both. The current rating is decided by the sense resistor and current capability of the voltage regulator being powered.

High Output Current Regulators Limited by Die size, package and thermal transfer Linear and Switching Voltage Regulator Fundamental (App Note) http://www.ti.com/lit/an/snva558/snva558.pdf Before  going into current scaling circuits, let’s see why do we need scaling circuits at all? Can’t we have voltage regulators which can support higher currents? Here  is the standard block diagram of a voltage regulator showing series pass element. The  selection of pass element will depend on the current it is supposed to carry. For  lower currents, the series pass element can be single transistor but for higher currents it has to be darlington pairs. For  higher current regulators, the drop-out voltage of the series pass element also increases which in turn produces heat and power dissipation. The  package of such single devices supporting higher currents are also limited by die size, and thermal transfer as highlighted by safe-operating curve. More  details on these, can be found in the highlighted application report.

Current Scaling – Parallel Regulators Safe Operating Curve Size and Cost High-Current Low-Noise Parallel LDO Reference Design http://www.ti.com/tool/TIDA-01232 Now  let’s talk about current scaling by paralleling regulators. This is a very well known technique to scale the output current. Multiple  regulators can be connected in parallel to achieve higher output current keeping constant output voltage. There  is a TI reference design (TIDA-01232) available for this configuration. The  shortcomings for such configurations are again safe-operating curves of regulator, the overall size and total cost of the solution.

Current Scaling – Paralleling MOSFETs (Positive Regulator) Low-impedance MOSFET Driver 𝐹𝑜𝑟 𝑉 𝐵𝐸 =0.8𝑉 𝐼 𝐿𝐷𝑂 =17𝑚𝐴 Current Sensing Here  we are showing a simple innovative technique to scale the output current keeping constant output voltage. TPS7A4701 is a positive voltage regulator whose output is set to 3.3V by feedback resistors. A  47 ohm series resistor is used as current sensing element to enable the scaling circuit. The voltage across 47 ohm resistor is governed by a base -to-emitter voltage of a PNP transistor. The maximum current through the regulator is calculated as VBE of PNP transistor divided by 47 ohms. So for VBE = 0.8V, current is 17mA . Once  the current increases beyond 17mA, the low-impedance MOSFET driver turns on all the MOSFETs together through gate resistors. The turning-on and off of the MOSFETs is taken care by diode and a PNP transistor. The currents through each MOSFETs is balanced by using ballast resistors of 470m ohms. For example, as shown in the TINA simulation, total current at the output is 6 amperes whereas each MOSFET carries 2 amperes.   The  division of current through MOSFETs is uniform and balanced. For example, for  1 ampere output current, each MOSFET carries around 331 milli-amperes and for 60  amperes output current, each MOSFET carries approximately 20  amperes.

Current Scaling – Paralleling MOSFETs (Negative Regulator) Current Sensing 𝐹𝑜𝑟 𝑉 𝐵𝐸 =0.6𝑉 𝐼 𝐿𝐷𝑂 =14𝑚𝐴 Low-impedance MOSFET Driver Now  let’s have a look at similar structure for negative regulator TPS7A3001. The concept is similar to what we have seen in previous slide. The  47 ohm resistor in series with the regulator decides the current flowing through regulator. A  PNP transistor in parallel with 47 ohm sets the current to approximately 14 milli-amperes. Once the output current crosses 14 milli-amperes, the  low-impedance MOSFET driver turns on all the MOSFETs at the same time. The current flowing through each MOSFET is balanced and equal.

Ballast Resistors – to take care of VGS effects VGS = 3V Drop across 0.47Ω VGS = 4V MOSFET VGS Total VGS required to turn-on MOSFET All drains are connected together All sources are connected together There  is one question we haven’t answered yet. How does ballast resistor help in balancing the current by reducing the effect of VGS for different MOSFETs? Let’s have a detailed look at the ballast resistors. As shown in the circuit, all  MOSFET drains are connected together and all MOSFET  sources are connected together. Each  MOSFET needs a voltage VGS to turn it on, which is added to the voltage  drop across ballast resistor. The total VGS required to turn on the MOSFET is sum of both these  parameters. Let’s  take numerical example to understand this. Hypothetically, let’s assume MOSFET  Q1 has VGS threshold equal 3V and Q2 has VGS threshold equal to 4V. So when the total supplied gate-voltage is increased from zero volts, it will turn on Q1 at 3 volts, while Q2 is still not turned on. There will be full  current passing through Q1 and R1, developing drop across 0.47 ohms. More the current, more is the drop across ballast resistor. This will ask for more total VGS value to keep the current flowing continuously. As we increase total VGS to 4 volts, it will also turn-on Q2 and the current  will also start flowing through Q2 path. Now both the branch carry same current. This effect of VGS is taken care dynamically, and is reduced as we have higher currents flowing through the MOSFETs.

Selecting MOSFET for Scaling Current CSD19533KCS 𝑉 𝐷𝑆 = 𝑉 𝐴𝑉𝐺 = 𝑉 𝐻𝐼𝐺𝐻 − 𝑉 𝐿𝑂𝑊 2 𝐼 𝐷 = 𝑇𝑜𝑡𝑎𝑙 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑁𝑜. 𝑜𝑓 𝑀𝑂𝑆𝐹𝐸𝑇𝑠 𝑖𝑛 𝑃𝑎𝑟𝑎𝑙𝑙𝑒𝑙 𝑉 𝐷𝑆 = 140−100 2 = 40 2 =20 𝑉 𝑃𝑎𝑐𝑘𝑎𝑔𝑒=𝐵𝑎𝑠𝑒𝑑 𝑜𝑛 𝑆𝑂𝐴 𝐶𝑢𝑟𝑣𝑒 𝐹𝑜𝑟 1𝑚𝑠 𝑝𝑢𝑙𝑠𝑒, 𝐼 𝐷 =10𝐴 Finally , it is important to understand selection of MOSFET in such scaling sceheme. The input of regulator typically has a capacitor which discharges linearly  when a burst of current is pulled from the output. Since all the MOSFET drains are connected together, VDS  equals the average voltage which is half of Vhigh – vlow. The  current through each MOSFET is total output current divided by number of MOSFETs used in the scaling circuit. The  package is based on the safe-operating curve of the MOSFET. For  example, for TI’s NextFET CSD19533KCS, if  VDS is equal to 20V, for 1 milli second output current pulse, the supported drain current is 10  amperes.

TIDA-01371 is available on TI.com Visit www.ti.com/tool/TIDA-01371 to find design resources (Gerbers, Schematics and more). This  voltage and current scaling scheme is implemented in TI Design TIDA-01371 avaialble on TI web. For more information and reading, visit www.ti.com.

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