1. High frequency Sine wave inverter
Challenges in voltage feedback

2. System Configuration DSC on primary (low voltage) side
Front end Switched mode isolated converter
Boost battery voltage to 400V
Hi bridge Inverter to generate sine wave
Objective
Control the output to a sine wave under various loads

3. Requirements of feedback circuit
High Voltage feedback
Isolation
Low cost /rugged
Linearity

4. RMS control mode Measure rms voltage err. @ (50Hz/60Hz freq)
Change ONLY modulation index ‘m’ with RMSV err
Duty d = m.sin(2.p.f.t) is
@ Steady state maintains sine wave
Poor /slow dynamic response approximately 100ms
Distortion under non linear loads like rectifier
Extremely low bandwidth
Low computation load req.
Low execution rate

5. RMS control mode feedback
Std Low grade/low cost 50Hz Potential Transformer(PT)
Std part in electric metering (widely available)
Satisfies 4 desirables
Isolation
Low cost
Ruggedness
50/60 Hz output voltage

6. Instantaneous control
Industry term for Hi bandwidth digital control
Hi sampling frequency and code execution rate ~ 20 KHz
Great dynamics approx.
~400 us step response
Non linear and fast changing loads handled
Large stiffness (low output impedance)
Perfect regulation even with leading /lagging PF

7. Feedback requirement Need
Accurate real-time fast high bandwidth voltage feedback
PT limitations
PT magnetically coupled device
No output of PT for DC component in input signal
Any DC offset will not be visible to DSC/DSP

8. PT output (DC offset blocked)
PT feedback characteristics
PT input with DC offset
PT output (DC offset blocked)
Low and High frequency signals undergo significant distortion

9. Challenge Using PT feedback would lead to… Output Voltage
Unstable or saturated intermittently (+/-400 Vdc)
Result
Malfunction of Downstream equipment
Damage to Downstream equipment
Catastrophic results or shutdown !!!!

10. output voltage and PT feedback are not correlated
Actual voltage: 200 V/div 
PT output 
output voltage and PT feedback are not correlated

11. Summary
The PT works properly 50/60Hz sine wave is being measured
To get accurate sine wave the PT needs to work properly in closed loop
Vicious dependency deadlock causes catastrophe !!!!
Chicken and egg problem !!!!

12. Proposed Solution No DC offset in output waveform
PT based feedback instantaneous control
Pure software solution
Low THD (slightly worse wave shape than ideal feedback)
Expensive, Complex ,Non linear isolation solutions not needed !!!
E.g. : Opto-couplers, voltage to frequency converters,
PWM generators
Patent pending

13. Experimental results Instantaneous closed loop control system
Voltage output
100 V/div
PT output
Final output voltage waveform

14. Thank You