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High Voltage and High Current Load Switch/Relay

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Newbie level 1
Feb 16, 2009
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load switching relay

Hello All,

I'm trying to find a Relay, preferably Solid State, that can handle delivering and switching high voltage/High current to and from a bank of capacitors.

To simply put it, i am trying to charge multiple capacitors (concurrently) with one set (3 bank) of relays and switch between each capacitor bank to discharge into a Flash Lamp with another set of relays (3 bank). Just like charging "Low Charge" batteries and choosing between them for controlling a device.

The spec's are these:

Charge side:

500Vdc Power supply to charge each capacitor and shut off
when desired voltage reached.
7Amp charge current into each capacitor until shutoff.

Discharge side:

Max 500Vdc on the capacitor into the flash lamp head.
Large Surge current for short time. No more then ~25A/Sec.

So essentially, there are 3 capacitors, 3 charge control relays, and 3 capacitor bank selectors. The control system is a microcontroller based system which monitors the charge and bank selector with either 5 volts or 12volts though a transistor. I've found and tried to use a lower dc voltage based contact relay, but when the voltage exceeds 300vdc, the discharge feeds back into the control coil from the surge of energy rushing through and resets the control system and with prolonged use, destorys the relay. I also have an inline fast-blow fuse of 20A with the discharge into the lamp head and does not break (shows an inrush of less then 20amps/sec).

I've searched the internet far and wide and found little to nothing of these specs. Although I have found Mosfets that support this voltage and current but am having difficulties trying to find a soltion in finding a way to make Mosfets work like a relay or load switch. FYI, there are two relays per capacitor, one for charging and one for bank selection, so i cannot do lowside driving with any switching circuit (Seperating ground from the capacitor). Highside driving seems to be the solution but seems to be difficult as the capacitor voltage rises the source voltage rises which means the gate voltage has to follow (rated voltage of +- 30VGS).

I'll post up a few ideas i have with the mosfets once I draw them out.

I'm hoping someone has anyidea of how to do this unique idea of switching high voltage/high current and speaks out. Thanks for your help!!


Full Member level 5
Oct 26, 2003
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high voltage load switch

A lot of manufactures make High Side drivers. Avago ,IR and Fairchild to name but a few.

Here are some of Avago’s offerings.


There are also photovoltaic drivers such as the VO1263 AND LH1262 these have peak output current in uA though.

I’m using the FAN7371


To switch a in a 70w flyback remotely (90 – 140 VAC in). I went this way because I want to consume as little current as possible in my drive circuit. I’m using a PIC to monitor the AC line and inhibit the driver until zero crossing is detected. The PIC also provides a charge pump for the boost capacitor on the driver, this way it can stay on continuously.

You can read up on an978 which shows bootstrap drivers and how to use them.


Slup169 also has info regarding HS driving.


The thing that you have to consider is that 600V mosfets with less then 0.5 ohm Rdson are expensive. You will likely need to parallel FETS $$. The cheapest 500V FET I could find less then 2 bucks is the STW14NK50Z 0.38 ohms Rdson. At my worst case Rms current the Fet will dissipate just over a watt. You would want at least a 600V Fet. You also have to be sure during turn on your FET stays in the SOA this is the highest stress with a large inrush current and the FET transitioning from off/on.

I’ve done some scaled down test on the FAN driver switching a 60nC FET blocking a rectified 37Vrms. The FET placed in between the bridge rectifier and the filter cap the cap is only 220uF. The FAN switches the FET in 260nS.

If money and current consumption is no object it’s simple buy a beefy driver multiple FETs and use an isolated supply for the driver.

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