1 EE462L, Spring 2014 Isolated Firing Circuit for H-Bridge Inverter (partially pre-Fall 2009 approach)

2 Isolation is needed because we have three separate MOSFET source nodes, and these three nodes are ground references for the respective firing circuits One logic signal toggles A+,A– One logic signal toggles B+,B– Vdc (source of power delivered to load) Load A + B + A – B – Local ground reference for A + firing circuit Local ground reference forB + firing circuit Local ground reference forB − firing circuit Local ground reference for A − firing circuit S S S S !

3 8 5 Comp kΩ V tri V cont –V cont 270kΩ 1kΩ 1.5kΩ V(A+,A–) –12V from DC-DC chip +12V from DC-DC chip Common (0V) from DC-DC chip +12V –12V Comparator Gives V(A+,A–) wrt. Common (0V) V cont > V tri V cont < V tri +24V 0V0V V cont > V tri V cont < V tri Use V(A+,A–) wrt. –12V Output of the Comparator Chip Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply

4 8 5 Comp kΩ V tri V cont –V cont 270kΩ 1kΩ 1.5kΩ V(B+,B–) –12V from DC-DC chip +12V from DC-DC chip Common (0V) from DC-DC chip +12V –12V Comparator Gives V(B+,B–) wrt. Common (0V) –V cont > V tri – V cont < V tri +24V 0V0V – V cont > V tri – V cont < V tri Use V(B+,B–) wrt. –12V Output of the Comparator Chip Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply

5 V(A+,A−) control signal V(B+,B−) control signal Reference (is −12V from DC-DC chip) The control signals at the open-circuited output of the PWM control circuit are +24V, or 0V When V(A+,A−) is 24V, MOSFET A+ is on, MOSFET A− is off When V(A+,A−) is 0V, MOSFET A+ is off, MOSFET A− is on MOSFETs B+ and B− work the same way with V(B+,B−)

6 0V +24V 0V +24V 0V +24V 0V +24V Look for symmetry of pulse centers

7 Onefiringcircuit for eachMOSFET, with each firing circuit mounted on a separate protoboard. Protoboards A – and B – can share a power supply and ground. A + and B + must each use separate power supplies and grounds. Do not connect any of these grounds to the ground of the control circuit. O + O – (see Figure 2 for connections) Powered by+12V that isisolated fromthe PWMcontrol circuit 10kΩ 0.1µF 10Ω 1.2kΩ MOSFET G D S 100kΩ 5 4 Opto Driver 8 1 Outline of protoboard A + and B + use inverting drivers (1426’s). A – and B – use non- inverting drivers (1427’s). The optocouplers provide an additional inversion. green g g blue for A +,B +, violet for A –,B – blue red blue Grounds(isolated from control circuit) Wait until next week Switching diode 14mA Optocoupler is current- controlled. Gate current turns on the transistor, which pulls down the collector voltage. Isolating barrier Once the MOSFET is connected, this asymmetrical circuit will add blanking by making the turn-on slower than the turn-off. (blanking is the opposite of overlap) Overlap is the time that A + and A − are simultaneously “on,” which should be avoided. Hence, some blanking (time between one turning off and the other turning on) is desirable.

8 14mA from control circuit 10kΩ +12V + V driver = 0V – Isolating barrier 14mA to Opto Input Yields ≈ 0V to Input of Driver Chip, so Inverting Driver Chip Turns MOSFET ON To driver 1.2mA (will pull down V driver to zero) Spec. sheet current transfer ratio  0.2 to 0.3 (times 14mA) !

9 10kΩ +12V + V driver = 12V – 0mA Isolating barrier 0mA to Opto Input Yields 12V to Input of Driver Chip, so Inverting Driver Chip Turns MOSFET OFF To driver 0mA from control circuit !

10 We use the control signals to send 14ma through optocouplers on each of the four firing circuit boards A+ and A− are daisy chained B+ and B− are daisy chained (for complementary outputs) So, each 14mA control signal passes through two optocouplers in series

11 24V control signals from the comparators, less 3.2V drop across two series optocoupler LEDs, and with 14mA, requires about 1.5kΩ of resistance in series with the daisy-chained optocouplers With 14mA, the LED of each optocoupler has about 1.6V drop If applied half the time, 24V across a 1.5kΩ resistor would produce about 0.2W. So, it is a good idea to size up to ½W resistors.

12 Thus, you use ½W series resistors between the comparator chip and the output terminals

13 Layout of inverter control circuit and isolated firing circuits A + A − B + B − No MOSFETs connected yet (i.e., the drivers are open-circuited)

14 Keep the 0.1µF capacitors across the drivers to prevent driver failure Use the same pattern for B+ and B– One DC converter chip feeds A+ Another DC converter chip feeds B+ Wall wart feeds A− and B− Zoom-in view of A+ and A– isolated firing circuits

15 wall wart input chip output − + − + Side view of A+ and A– isolated firing circuit and single 12V isolated DC-DC converter chip that powers A+ Socket each single DC-DC converter chip, using one half of an 8-pin SIP socket. Carefully break an 8-pin SIP socket in half. Do this by clamping on one-half with your long-nose pliers, and then bending the other half down with your fingers. It should be a clean break.

16 V(A +,A – ) Opto A + output Save screen snapshot #1 0.5V 3.2V Input and Output Voltages of Optocoupler V cont = 0 (i.e., m a = 0) in this Snapshot 12V 0V Opto Input (the 1.5kΩ resistor drops the voltage from 24V to 3.2V) As expected, the opto output is inverted This phototransistor turn off delay will limit your PWM operating frequency Phototransistor turning on Phototransistor turning off Look for Symmetry Among all Four Circuits ! Different time-constants to avoid shoot-through (i.e. to provide a “dead-time”)

17 Look for Nearly Perfect Alignment Between V(A+,A−) Signal to Optocoupler, and Output of A+ Inverting Driver Chip V(A +,A – ) A + driver output In phase

18 Look for Nearly Perfect Out of Phase Alignment Between V(A+,A−) Signal to Optocoupler, and Output of A− Non-Inverting Driver Chip V(A +,A – ) A – driver output Out of phase

19 Now the present circuit based on PCBs:

20 Key new component: IRS21844

21 IRS21844 High output Low output Actual pinout

22 IRS21844 Blanking time and isolation already integrated in a single IC