1. High frequency Sine wave inverter
Challenges in voltage feedback
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copyright 2011 controltrix corp www. controltrix.com

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
copyright 2011 controltrix corp www. controltrix.com

3. Requirements of feedback circuit
• High Voltage feedback
• Isolation
• Low cost /rugged
• Linearity
copyright 2011 controltrix corp www. controltrix.com

4. RMS control mode
• Measure rms voltage err. @ (50Hz/60Hz freq)
• Change ONLY modulation index ‘m’ with RMSV err
• Duty d = m.sin(2. .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
copyright 2011 controltrix corp www. controltrix.com

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
 Linearity @ 50/60 Hz output voltage
copyright 2011 controltrix corp www. controltrix.com

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
copyright 2011 controltrix corp www. controltrix.com

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
copyright 2011 controltrix corp www. controltrix.com

8. PT feedback characteristics
PT input with DC offset PT output (DC offset blocked)
• Low and High frequency signals undergo significant distortion
copyright 2011 controltrix corp www. controltrix.com

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 !!!!
copyright 2011 controltrix corp www. controltrix.com

10. Actual voltage: 200 V/div
PT output
output voltage and PT feedback are not correlated
copyright 2011 controltrix corp www. controltrix.com

11. Summary
• The PT works properly only @ 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 !!!!
copyright 2011 controltrix corp www. controltrix.com

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
copyright 2011 controltrix corp www. controltrix.com

13. Experimental results
Instantaneous closed loop control system
Voltage output
100 V/div
PT output
Final output voltage waveform
copyright 2011 controltrix corp www. controltrix.com

14. Thank You
consulting@controltrix.com
copyright 2011 controltrix corp www. controltrix.com