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I wanted to make a circuit to produce square waves for the transformer. This circuit uses a bootstrap driver for the gate of the MOSFETs. I know the design is very poor but that's just a rough schematic from my mind. This is basically an inverter circuit that produces a +-300V at the output (going to give it to transformer). Practically I will use IGBTs instead of MOSFETs.

enter image description here

I have a few questions in this circuit:

  1. I was wondering if it is possible to make a bootstrap at the lower side. Actually I wanted to avoid the use of -300V at the transistor end and wanted to make the driver consisting of relative lower voltages.

  2. Secondly I had seen drivers such as the M57962AL and there is a push-pull configuration of transistors there, shown below. I couldn't add that in my configuration. kindly tell me if anything is wrong/how can I add that.

enter image description here

Kindly ignore the high/lows on the high side of the mosfets. The logics are inverted. The voltage shown in the graph is the one applied between the resistor.

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2 Answers 2

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There are some issues in the circuit. The bootstrap capacitor will charge to 315 V when low side output MOSFET is activated, so it will destroy high side MOSFET`s gate. The negative terminal of power supply +15 V for bootstrapping must be connected to -300 V supply. Also the low side level shifter will force VGS of low side MOSFET to 300 V when it is activated.

See example of level shifter with BJT, and auxiliary power supply +15V connected to DC -300V. If this PSU is an issue, search for charge pump to get these voltage, or use optocouplers. Transistors for level shifter (Q6 and Q7 of example) must support more than 300 V.

enter image description here

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  • \$\begingroup\$ Are you using the transistors to simply short and unshort the gate and the source of the MOSFETs upon getting 0/5V on the Q8/Q6? this way the gate to source is getting a zero voltage drop. \$\endgroup\$
    – kam1212
    Sep 10, 2023 at 12:22
  • \$\begingroup\$ @kam1212: not sure to understand your comment. Q6/Q8 will work with positive voltage from ground, maybe 5 V or more, depends on base resistor. At the collector of level shifter transistor Q7 there is a resistor divider, this should be ajusted to get at R8 the gate-source voltage required. Totem pole (Q1/Q2 and Q3/Q4) will amplify (to high and low signal) current to drive gate, so you need less current in previous stage. \$\endgroup\$
    – Bravale
    Sep 11, 2023 at 7:51
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The first question I would like to ask about the circuit (as drawn now): Is the node marked as "-300V" really 300V below 0V (0V is the ground node), or is it just the name given to the negative terminal of the 300VDC supply? It would greatly help to show how the nodes marked "+300V", "-300V, and "+15V" are related to the reference node ("0V", ie: the ground symbol).

The next question is: what are the identifiers for each of the components? None of the components can be identifed by a simple string, eg: "R1", or "Q1". This makes it very difficult for anyone trying to help you, since we cannot easly refer to a partuclar commpionent on your diagram. For example, "Q1" is much easier and clearer than writing: "the left-most MOSFET with its drain connected to the +300V node".

To try to assist you with this: For an inexperienced designer, I recommend not trying to design your own "home-brew discrete boot-strap" circuit for an application where the main power supply rails are more than about 50V above or below the control circuit (ground). There are just too many details and things that can go wrong with boot-straps operating over 50V above the control circuit, and the higher the voltage the more issues there are to worry about.

Instead, I would recommned using a more complete solution, such as:

  1. A commercially available solution using the bootstrap technique, for example:
    https://www.digikey.com.au/en/products/detail/infineon-technologies/IR2110PBF/811566
    https://au.mouser.com/datasheet/2/472/si823x_datasheet-3084970.pdf
    https://rocelec.widen.net/view/pdf/ttqyaoj0x5/ANDIS03903-1.pdf?t.download=true&u=5oefqw

  2. Using an IC designed to work with isolation transformers, eg:
    https://www.st.com/resource/en/application_note/an461-an-isolated-gate-drive-for-power-mosfets-and--igbts-stmicroelectronics.pdf

There are many resources to explain the various design considerations of gate drivers for high-voltage (>50V) applications, here is just one example:
https://www.onsemi.com/pub/collateral/tnd379-d.pdf

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