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I've been looking at driving an H-Bridge to raise the frequency of the mains voltage from 60 hz to somewhere between 60-120 hz.

I'm planning to use an Atmel Atiny84 to generate my clock frequency to drive an H Bridge. The mains voltage will be rectified by a full bridge rectifier, giving me nominally 240 VDC at up to 50 amps.

I have scavenged basically all the parts needed to do this, but from what I understand the highside of the H-Bridge requires either the use of P-channel MOSFETs or a voltage higher than the source voltage. This is because Vgs determines the conduction state of the MOSFET. So if the source is being fed at approximately +240 VDC, I need at least +252 VDC for most power MOSFETs to be saturated and fully conducting.

Most commercial circuits use a "charge pump" built into a large brick of silicion that spec. sheets are not available on. So my question is if I already have 240 VAC, can I just feed any of the common voltage doubler circuits and wind up with 480 VDC? The current I need for the gate is small, so capacitor size should not be an issue. I'd wind up with +240 V for Vgs if I did this, which is way too high for common power MOSFETs. But can I somehow feed this very high voltage into a common part like a 7815 to wind up with a Vgs of approximately +15 V ? This might require more components and be less effecient, but I don't really care.

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  • \$\begingroup\$ Maybe this is what you need fairchildsemi.com/datasheets/FA/FAN7382.pdf \$\endgroup\$
    – Swarles Barkely
    Commented Feb 16, 2016 at 4:47
  • \$\begingroup\$ Note also your DC voltage will be either 170V or 340V, but in no case 240V... \$\endgroup\$
    – user16324
    Commented Feb 16, 2016 at 11:35
  • \$\begingroup\$ @BrianDrummond Without load, the capacitors should charge to the line voltage * sqrt(2). The DC voltage will have significant ripple however, so its still nominally 240 VDC. \$\endgroup\$
    – Eric Urban
    Commented Feb 16, 2016 at 14:36
  • \$\begingroup\$ No. The caps will charge to 340V and decay a few volts (certainly not 100V) before the next peak. However this is necessary to generate 240V rms sinewave output. (presumably via PWM). It doesn't affect your question (see Andy and Rob's good answers) but will affect your component ratings. \$\endgroup\$
    – user16324
    Commented Feb 16, 2016 at 14:48

2 Answers 2

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If you need to raise the voltage for your signal to trigger an N-channel MOSFET on the high-side of your bridge, you can use any isolating DC-DC converter to output 12VDC(isolated) & tie the converter's output "GND" to the +240V of the line you're wanting to switch.

This places the +12V output 12V above the "virtual ground"of your +240VDC line, thus providing your 252V voltage for driving the N-MOSFET(s).

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You can probably use a simple AC transformer with secondary bridge rectifier and smoothing capacitor. The secondary output is isolated from the primary by virtue of the gap/insulation and so it's fairly reasonable to expect that the secondary can be "lifted" to the 240V DC you already have. Something like a 9V AC secondary will produce about 12V DC after rectification and smoothing.

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  • \$\begingroup\$ That might work, I have an entire bag of small transformers (approx 5VA) that I have removed from appliances. I'll have to sketch out a circuit and see if it seems reasonable. \$\endgroup\$
    – Eric Urban
    Commented Feb 16, 2016 at 14:37

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