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Old 8th June 2011, 12:26 AM   #1
dgta is offline dgta  United States
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Join Date: May 2011
Default High voltage switching

Looking for possible solutions to switching B+ for delay purposes. Need to switch 470-500vdc supplies in several amps, all CT secondaries.

Has anyone come up with a simple HV switching scheme that has proven robust and reliable? MOSFET, IGBT, one of the new MOS optorelays? I'm not adverse to mechanical relays, but I don't know any compact ones with contacts rated for 500vdc.
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Old 8th June 2011, 12:38 AM   #2
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If you go for mechanical type relays then you need to switch the AC ahead of the rectifier(s). Either switch both sides of the HV winding or simply switch the CT to 0V.

Switching the DC should be petty simple using a MOSFET and an Opto-coupler to switch the gate to the drain via a suitable (say 220K) resistor. This is a good solution as multiple HV supplies can switched at the same time by simply running a (say 5mA) low voltage current loop through all of the opto-coupler input LEDS. Dont forget to add a gate to source protection zener and a gate stopper resistor to the MOSFETs. That current loop can be driven directly from the output of the delay on timer circuit (555 timer or similar).

Cheers,
Ian

Last edited by gingertube; 8th June 2011 at 12:40 AM.
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Old 8th June 2011, 10:46 AM   #3
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One of my 5 minute design slow turn-on circuits using an upside down mosfet in the rectifier AC neg leg and the negative bias supply to actuate a small relay as a self protect B+ shutdown in fixed bias systems. Mosfet selected for max voltage.

This circuit works well but haven't tried it with a power IGBT, but the concept should work for other voltages with a bit more workin' out. Those round circles in mosfet legs are ferrite beads. I love'm.

richy
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Old 8th June 2011, 12:30 PM   #4
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richy - have you checked what time duration and peak current level pass through the fet during soft turn-on, and how that compares with SOA? Ciao, Tim
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