SG3525 Full Bridge Inverter Circuit

Mar 18, 2017

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 complex.

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 mosfets.

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 inverter.

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 unfamiliar.

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 mosfet.
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!


  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.

  2. Sir I think your high side and low side GATEs are incorrect position or connection. Am I right?

    1. Nope, I have already explained the reason in the above comment....

  3. 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. Hello Prathamesh, which cirucit are using using? is it using the IRS based full bridge driver? please provide the schematic details....if it's an IRS based driver, then yo can use its SD (shut down pinout for the protection feature...for other info you can study the following article:



Search This Blog