High frequency Sine wave inverter Challenges in voltage feedback www.controltrix.com

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

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

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

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

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

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

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

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 !!!!

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

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 !!!!

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

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

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