IGBT control in a buck-boost DC-DC converter

Reader Jil Sutaria wrote in a with a query about controlling four IGBTs in a buck-boost DC-DC converter. Some of the main features of this application are:

• Two pairs of IGBTs need to be controlled.
• Each pair of IGBTs share one “leg” of what looks like a H-bridge. Each pair is driven in complementary mode (i.e. one IGBT is off whenever the other is on). I’m guessing there should probably be some dead time at the point where the two devices swap over, but for the time being I’m ignoring this.
• Both pairs of IGBTs have the same duty cycle – 73% in boost mode and 27% in buck mode.
• The two pairs are driven 180 degrees output phase.
• The switching frequency is 20kHz.

I’ve sketched out an example program. It’s really still a work in progress, since the waveforms don’t change at all – they just constantly repeat the same pattern. However, they’re at least coming out about the right shape (see waveform graph below).

//
// igbt_dcdc_converter.c
// dsPIC30F4011 program for IGBT DC-DC converter
//
// Written by Ted Burke
// Last updated 4-1-2013
//

#include <xc.h>
#include <libpic30.h>

// Configuration settings
_FOSC(CSW_FSCM_OFF & FRC_PLL16); // Fosc=16x7.5MHz, Fcy=30MHz
_FWDT(WDT_OFF);                  // Watchdog timer off
_FBORPOR(MCLR_DIS);              // Disable reset pin

int main()
{
	// Make all port D pins outputs
	TRISD = 0;
	
	// Configure timers 2 and 3 for 20kHz,
	// but 180 degrees out of phase
	PR2 = 1500; TMR2 = 0;
	PR3 = 1500; TMR3 = 750;
	
	// Configure output compare channels 1 & 2
	// to control one pair of IGBTs (using Timer 2)
	OC1CONbits.OCM = 0b101; OC1CONbits.OCTSEL = 0;
	OC2CONbits.OCM = 0b101; OC2CONbits.OCTSEL = 0;
	OC1R = 0; OC1RS = 0.73 * PR2;
	OC2R = 0.73*PR2; OC2RS = PR2;
	
	// Configure output compare channels 3 & 4
	// to control the other pair of IGBTs (using Timer 3)
	OC3CONbits.OCM = 0b101; OC3CONbits.OCTSEL = 1;
	OC4CONbits.OCM = 0b101; OC4CONbits.OCTSEL = 1;
	OC3R = 0; OC3RS = 0.73 * PR3;
	OC4R = 0.73 * PR3; OC4RS = PR3;
	
	// Enable both timers to start waveforms
	T2CONbits.TON = 1;	// Enable timer 2
	T3CONbits.TON = 1;	// Enable timer 3
	
	// Now just hang around here and let the
	// output compare pins do all the work!
	while(1);
	
	return 0;
}

Here’s a quick graph I captured from the pins using my makeshift logic analyser (see next post). The waveforms are at least roughly the right shape and the desired 180 degree phase shift seems to be working correctly.

The x-axis values are sample numbers. My makeshift logic analyser is sampling four channels in parallel at a sampling frequency of approximately 1MHz. I say approximately because I’ve actually just done it roughly with a 1us delay function (specified as 30 instruction cycles – the dsPIC is running at 30 MIPS) rather than using an interrupt. The real sampling period is probably slightly longer than the intended value and there’s probably a slightly longer inter-sample period every fourth sample due to the way the sampling is done in a loop. Also, I’m using the internal fast RC oscillator to drive my dsPIC, so that’s probably introducing some additional timing variability.

The y-axis values are basically meaningless – I just added an offset to each channel to spread them out on the graph. Each output compare channel is actually switching between 0V and 5V. The channels are displayed in the following order from bottom to top on the graph: OC1, OC2, OC3, OC4.