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SG3525 Full Bridge Inverter Circuit

In this post we try to investigate how to design a SG3525 full bridge
inverter circuit by applying an external bootstrap circuit in the
design. The idea was requested by Mr. Mr. Abdul, and many other avid
readers of this website.
Whenever we think of a full bridge or an H-bridge inverter circuit,
we are able to identify circuits having specialized driver ICs which
makes us wonder, isn’t it really possible to design a full bridge
inverter using ordinary components?

Although this may look daunting, a little understanding of the
concept helps us realize that after all the process may not be that

The crucial hurdle in a full bridge or a H-bridge design is the
incorporation of 4 N-channel mosfet full bridge topology, which in turn
demands the incorporation of a bootstrap mechanism for the high side

So What’s exactly a Bootstrapping Network and how does this become so crucial while developing a Full bridge inverter circuit?

In one of my earlier posts I comprehensively explained how emitter follower transistor works, which can identically true for a mosfet source follower circuit.

In this configuration we learned that the base voltage for the
transistor must be always 0.6V higher than the emitter voltage at the
collector side of the transistor, in order to enable the transistor to
conduct across collector to emitter.

If we interpret the above for a mosfet, we find that the gate voltage
of an source follower mosfet must be at least 5V, or ideally 10V higher
than the supply voltage connected at the drain side of the device.

If you inspect the high side mosfet in a full bridge network, you
will find that the high side mosfets are actually arranged as source
followers, and therefore demand a gate triggering voltage that needs to
be a minimum 10V over the supply volts.

Once this is accomplished we can expect an optimal conduction from
the high side mosfets via the low side mosfets to complete the one side
cycle of the push pull frequency.

Normally this is implemented using a fast recovery diode in conjunction with a high voltage capacitor.

This crucial parameter wherein a capacitor is used for raising the
gate voltage of a high-side mosfet to 10V higher than the supply voltage
at its drain is called bootstrapping, and the circuit for accomplishing
this is termed as bootstrapping network.

The low side mosfet do not require this critical configuration simply
because the source of the low side mosets are directly grounded and
therefore are able to operate using the supply voltage itself and
without any enhancements.

How to Make a SG3525 Full Bridge Inverter Circuit

Now since we know how to implement a full bridge network using
bootstrapping, let’s try to understand how this could be applied for
achieving a full bridge SG3525 inverter circuit, which is by far one of
the the most popular and the most sought after ICs for making an

The following design shows the standard module which may be
integrated to any ordinary SG3525 inverter across the output pins of the
IC for accomplishing a highly efficient SG3525 full bridge or H-bridge
inverter circuit

Referring to the above diagram, we can identify the four mosfets
rigged as an H-bridge or a full bridge network, however the additional
BC547 transistor and the associated diode capacitor looks a bit

To be precise the BC547 stage is positioned for enforcing the
bootstrapping condition, and this can be understood with the help of the
following explanation:

We know that in any H-bridge the mosfets are configured to conduct
diagonally for implementing the intended push pull conduction across the
transformer or the connected load.

Therefore let’s assume an instance where the pin#14 of the SG3525 is
low, which enables the top right, and the low left mosfets to conduct.

This implies that pin#11 of the IC is high during this instance,
which keeps the left side BC547 switch ON. In this situation the
following things happen withing the left side BC547 stage:

1) The 10uF capacitor charges up via the 1N4148 diode and the low side mosfet connected with its negative terminal.

2) This charge is temporarily stored inside the capacitor and may be assumed to be equal to the supply voltage.

3) Now as soon as the logic across the SG3525 reverts with the
subsequent oscillating cycle, the pin#11 goes low, which instantly
switches OFF the associated BC547.
4) With BC547 switched OFF, the supply voltage at the cathode of the
1N4148 now reaches the gate of the connected mosfet, however this
voltage is now reinforced with the stored voltage inside capacitor which
is also almost equal to the supply level.
5) This results in a doubling effect and enables a raised 2X voltage at the gate of the relevant mosfet.
6) This condition instantly hard triggers the mosfet into conduction,
which pushes the voltage across the corresponding opposite low side
7) During this situation the capacitor is forced to discharge quickly
and the mosfet is able to conduct only for so long the stored charge of
this capacitor is able to sustain.
Therefore it becomes mandatory to ensure that the value of the
capacitor is selected such that the capacitor is able to adequately hold
the charge for each ON/OFF period of the push pull oscillations.
Otherwise the mosfet will abandon the conduction prematurely causing a
relatively lower RMS output.
Well, the above explanation comprehensively explains how a
bootstrapping functions in full bridge inverters and how this crucial
feature may be implemented for making an efficient SG3525 full bridge
inverter circuit.
Now if you have understood how an ordinary SG3525 could be
transformed into a full fledged H-bridge inverter, you might also want
to investigate how the same can be implemented for other ordinary
options such as in IC 4047, or IC 555 based inverter circuits, …..think
about it and let us know!

SG3525 Inverter Circuit which can be Configured with the the above Discussed Full Bridge Network

The following image shows an example inverter circuit using the IC SG3525, you can observe that the output mosfet stage is missing in the diagram, and only the output open pinouts can be seen in the form of pin#11 and pin#14 terminations.

The ends of these output pinouts simply needs to be connected across the indicated sections of the above explained full bridge network for effectively converting this simple SG3525 design into a full fledged SG3525 full bridge inverter circuit or an 4 N channel mosfet H-bridge circuit.

Feedback from Mr. Robin, (who is one of the avid readers of this blog, and a passionate electronic enthusiast):

Hi Swagatum
Ok,just to check everything is working I separated the two high side fets from the two low side fets and used the same circuitry as:
connecting the cap negative to the mosfet source then connecting that junction to a 1k resistor and an led to ground on each high side fet.Pin 11 pulsed the one high side fet and pin 14 the other high side fet.
When I switched the SG3525 on both fets lit up momentarily and the oscillated normally thereafter.I think that could be a problem if I connected this situation to the trafo and low side fets?
Then I tested the two low side fets,connecting a 12v supply to a (1k resistor and an led) to the drain of each low side fet and connecting the source’s to ground.Pin 11 and 14 was connected to each low side fets gate.
When I switched the SG3525 on the low side fet’s would not oscillate until I put a 1k resistor between the pin (11, 14) and the gate.(not sure why that happens).

Circuit diagram attatched below.

My Reply:

Thanks Robin,

I appreciate your efforts, however that doesn’t seem to be the best way of checking the IC ‘s output response…

alternatively you can try a simple method by connecting individual LEDs from pin#11 and pin#14 of the IC to ground with each LED having its own 1K resistor.

This will quickly allow you to understand the IC output response….this could be done either by keeping the full bridge stage isolated from the two IC outputs or without isolating it.

Furthermore you could try attaching a 3V zeners in series between the IC output pins and the respective full bridge inputs…this will ensure that false triggering across the mosfets are avoided as far as possible…

Hope this helps

Best Regards…

From Robin:

Hi Swagatam

Could you please explain how{ 3V zeners in series between the IC output pins and the respective full bridge inputs…this will ensure that false triggering across the mosfets are avoided as far as possible…

Cheers Robin


Hi Robin,

When a zener diode is in series it will pass the full voltage once its specified value is exceeded, therefore a 3V zener diode will not conduct only as long as the 3V mark is not crossed, once this is exceeded, it will allow the entire level of voltage that’s been applied across it
So in our case also, since the voltage from the SG 3525 can be assumed to be at the supply level and higher than 3V, nothing would be blocked or restricted and the whole supply level would be able to reach the full bridge stage.

Let me know how it goes with your circuit.


34 thoughts on “SG3525 Full Bridge Inverter Circuit

  1. Hello sir in this post the pulse from the ic is not configured in a diagonal form as your other full bridge configurations , is this oversite or the correct configuration for this sequence?

    1. Hello Ugoeze, it's because of the presence of the BC547 transistors which invert the signal for the upper mosfet making sure that the only the diagonal mosfets are triggered at any instant during the osculations.

    1. Thank you sir. I got now the idea of the bootstrapped implemented in your diagram. I already made your previous 1kw full bridge inverter and its fully functional. I'm now using it in my house to drive my appliances.

      I will test this new inverter diagram and I will let you know sir.

      Thank you very much sir for being helpful to us as a hobbyist.

    2. Hello sir.. good news! I finally build your design, and it's fully working. The output from the MOSFETs is 13.5vac using 12vdc/100ah battery. The output from the transformer is 240vac. My question is why 13.5vac output but my supply is 12vdc.?

      I'm using IRF3205, my transformer build is 220v-12v with 60hz.

    3. That's great silverman…I appreciate it a lot.

      where exactly did you find 13.5V?

      check it across drain/source terminals of the mosfet, you wlll find it to be around 6V,,,,that's the average value due to the ON/OFF cycles of a 12V supply

    4. Thanks for the reply sir. My bad.. I forgot my basic voltage check. I put my test probe across to the connector of 12v windings of transformer and I got AC readings at 13.5vac.

      It is now partially completed inside the wooden box. And I also made a changeover circuit and I Incorporate it to your design.

    5. I checked again the drain/source as instructed, the drain/source voltage is 6.75V. I double checked the schematics and it's properly oriented. And it's all components are soldered in PCB.

  2. Hello Sir,
    I am working on Full Bridge Inverter Circuit using IGBTs where the DC Voltage is not constant at the input, I wanted to know about the protection circuits which must be designed to avoid any damage.

    1. Thank you Sir, the link was very helpful. Also needed some help with Overvoltage and Overcurrent Protection Circuits. Will you please tell me the devices which must be used to protect the inverter from sudden fluctuations where the output of the inverter is 1.4kW with a nominal input voltage of 200V and maximum input voltage 250V?
      I'd appreciate any information you could give me.

    2. Prathamesh, since the inverter is being operated using a battery there cannot be any fluctuations, but an overload (overcurrent) could cause problems, for which you can use the shut down pinout of your full bridge IC to protect the system.

  3. Hi Swagatam
    Thanks for getting back to me in the pure sinewave inverter post.
    I would like to know in this full bridge circuit above,if i have a 290v dc supply voltage from a high frequency source if the 10uf 50v capacitor must be changed to a higher voltage and if the irf540 must be changed as well to handle the higher voltage.I want to switch this high voltage DC to 220vac at 50Hz
    regards Robin

    1. Hi robin, as far as I think, the mosfet drains will need to be connected with the 290V DC, while the diode anode separately connected to the 12V supply…yes the mosfet will need to replaced with a 400V type for example IRF804 etc…

    2. Hi Swagatum
      Ok on first attempt i burnt one of the IRF840's,so I'm just making sure that the resistor between pin 5 and 7 on the SG2535 causes a delay which it does.Also is'nt it a good idea to put a 1k resistor between the gate and source of each mosfet to make sure that it is not accidently on when you switch on,or would it affect the functioning of your circuit

    3. Hi Robin, yes adding a slight delay or dead time will help to control mosfet burning..

      you can add a resistor between gate and source if the mosfets are relatively away from the IC or the trigger source…

      no it won't affect the above explained circuit…and please note that the BC547 emitters are only connected to the ground and not to the mosfet sources

    4. Hi Swagatum
      I've been wrestling with this H-bridge circuit above and it worked the first time i made it,I slowed down the hz so i could see it oscillating,the moment i put a load on (bang).
      Ok,I've now disected it and first got the two high side Irf840's blinking,(source,1k,led,ground),pin 11 to the one IRF gate and pin 14 to the other but as soon as i switch the sg3525 on both led's come on at the same time thereafter it oscillates normally,would'nt this be a catastrophic occurance?.By the way the cap seems to have charged up via the load because i was getting 18v on the gates.
      Then i got the low side IRF's blinking(+12v,1k,led,drain,source,ground)pin 11 to 1k resistor then to gate,pin 14 same to other IRF but they would not oscillate without the 1k to the gate?
      Oh! I saw your comments with Bob the Blogger(interesting)
      Regards Robin

    5. Thanks for the update Robin,

      sorry I could not get this: (source,1k,led,ground)
      LED can be connect in series wit the BC547 base and across gate/source of the low side mosfet any other position could create problems wit the functioning or misleading indications.

      Please let us know, if possible through images if the deign works for you correctly as proposed i the article…

      wish you all the best..

  4. Hello swag, can this circuit be driven directly with a 24vdc supply?secondly , can the same transformer be used to charge the batterywithout any damage to the MOSFETs? Then thirdly why are the MOSFETs not having gate resistor like 10 ohms or any other thank .Karlas is my name and a follower of this blog .

  5. Hello swag.my questions are simple, first. Can this h-bridge circuit be driven directly with a 24v battery system, secondly. Can the transformer be used to charge the batteries on utility mode. Then thirdly why are the MOSFETs not having gate resistor like other circuit s do. Thanks that all for now.

    1. Hello Karlas, yes you can use 24V for the above shown H-bridge configuration.

      The transformer can be used only in the inverter mode, it cannot be used for charging the battery unless some special changeover circuitry is incorporated.

      the upper mosfets do not need gate resistor since the BJT's 1K itself forms the gate resistor for the mosfets…the lower mosfet could be added with a some low value gate resistor….although it's never too important.

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