Newbie
Joined: 04/06/2025
Location: AustraliaPosts: 2
| Posted: 04:25am 07 Jun 2025 |
Thanks for the replies. Appreciate the time you both have taken.
@phil99. From the scope shots KissIS posted, it appears that the IGBT conducts for between 20ms and 50ms with an equal off time. The safe operating area graph for the IKW30N60T is figure 2 on the datasheet. I used the DC graph as it was the closest to the operating conditions of the heater. The DC graph, as I understand it, is not operation in linear mode. The device is being driven fully on. The next graph in Figure 2 is for a 10ms pulse. I can't find what is the off time in the datasheet. So to be pedantic I should interpolate between DC and the 10ms pulse graphs. It makes little difference though. At 10ms, the maximum current is about 1A. I don't understand how you get a rating of 7.8A at 187V. This implies a pulse length of something between 50us and 1ms according to the SOA graph.
I agree that there is a switching loss each time the IGBT is turned on or off. This is reduced by decreasing the time needed to charge capacitance of the IGBT and is the role of the FOD3182. So why the 10R resistors in the gate circuit? Would they not simply increase the time needed to charge the gate capacitance and increase the switching loss?
If the FOD3182 drives the IGBT fully on the most significant loss would be the roughly 2V drop across the collector-emitter junction of the IGBT. If each IGBT carries 2.5A each, it would dissipate 5W.
So it seems to me that using a smaller capacitor and switching it faster should work. I'm looking for a second hand capacitor but if I can't find one, motor start capacitors are about $40 against $150 for a 4700uF part.
@KeepIS. The depth of the capacitor discharge was in the thread. From your scope shots and measurements it about a third.
I have a second hand 135L water tank and about 3KW of panels but no inverter. I've plumbed the electric water heater into the water inlet of a gas hot water heater which feeds the house. The idea is to preheat the water before the gas unit and save on gas. Hence the interest in this circuit.
It's obvious that the circuit works. I just wish I knew how it works so I could vary the components if need be.
Thanks again to both of you.