H bridge short circuit protection

You can learn how to build h-bridges from many on- and off-line resources. After all these circuits are not terribly complicated. Some of those resources are good, some are not so much. While the current material is based on those articles, it corrects many errors and is expanded and updated greatly. My plan is to eventually expand these articles to cover not just h-bridges but control circuits and electromechanical systems as well.

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration:. The switching elements Q Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this discussion. The diodes D D4 are called catch diodes and are usually of a Schottky type. The top-end of the bridge is connected to a power supply battery for example and the bottom-end is grounded.

h bridge short circuit protection

In general all four switching elements can be turned on and off independently, though there are some obvious restrictions. Though the load can in theory be anything you want, by far the most pervasive application if H-bridges is with a brushed DC or bipolar stepper motor steppers need two H-bridges per motor load. In the following I will concentrate on applications as a brushed DC motor driver.

The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards. In a bridge, you should never ever close both Q1 and Q2 or Q3 and Q4 at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply.

Because of this restriction from the four possible states the side-A switches could be in only three make sense:. This model will not be useable for control applications, where you try to electrically compensate for the effects of mechanical components. The main assumption in the model introduced here is that the mechanical time-constants in your system are much higher than the electrical ones, in other words we can consider the shaft speed to be constant for our analysis.

A DC motor is an energy conversion device: it takes electrical energy and turns it into mechanical energy. When operated as a generator, it does the opposite: converts mechanical energy into electrical.

In this very simple motor model, the mechanical parameters are completely ignored. On the electrical side, the motor basically contains a number of inductors, that move in a magnetic field. The inductors themselves of course have an inductance, and some internal resistance. Their movement in the field will generate a voltage — called generator voltage and denoted by V g — across the inductors. From this description, the following model can be drawn:.

In fact in many cases, the internal resistance of the inductors can be disregarded, and an even simpler model, an ideal inductor in series with a voltage source can be used:. In both cases, all the elements are in series, so the share the same current, but the voltage across them of course is different. The generator voltage V g depends only on the speed by which the inductors move in the field, in other words on the rotational speed of the motor.

The force or torque in a rotational system, like a DC motor these electromagnets — inductors — exert is proportional to the current flowing through them. If less than full-speed operation is intended the switches are controlled in a PWM fashion. It is a periodic signal, with a constant frequency. The information content — that is used to change the operating parameters of the bridge — is the ratio between the on-time and the off-time. The various drive modes differ in how the switches are set during the on-time and the off-time.

If we want the motor to do anything interesting, we will have to connect it to the power supply in at least one of the phases. We have two choices: either we turn on Q1 and Q4 or we turn on Q2 and Q3. So, whenever the bridge changes state with the motor current being non-zero, the new state has to make sure that the current can continue to flow in some way.Remember Me?

Short circuit protection in H-bridge. Short circuit protection in H-bridge I my attached circuit I want to limit it to 3A. Good and bad is that a 5 milliohm sense resistor only drops 15mV. Could include a shutdown at or over 3A using a comparator instead of the error amplifier idea, but that senses something more sensible than 15mV, maybe using a 0.

If you use a 0. Maybe hard to put together: a comparator that detects 3A and instead of shutting down triggers an error amplifier to limit current to the 3A value - not sure how to put that idea into practice without a lot of thinking, sorry.

If that's too large a resistor, then you need to go with a more complex circuit such as the comparator and transistor that d suggested. What's your preference? Zapper Curmudgeon Elektroniker. Re: Short circuit protection in H-bridge. Originally Posted by crutschow. A simple current-limit design uses two BJTs, but that requires about a 0. Originally Posted by d Originally Posted by KlausST. Originally Posted by BradtheRad. Simple method to measure current, referenced to the positive supply.

A PNP transistor is operated in common base mode. The 4 m-ohm shunt is placed at the top of the load. Last edited by crutschow; 16th December at The polarity on the PNP is incorrect for it to conduct base and emitter need to be reversed but, even if it did, with that value of shunt resistance the minimum short circuit current is about A!

For a 3A limit the sense resistor needs to be about 0. Re: Short circuit protection in H-bridge I'm puzzled as to how it can work. As the current increases the emitter voltage becomes more negative less positive when tends to shut the transistor off.

Or is that what you intended, that the transistor is normally ON and turns off as the current increases. If so, that circuit will be very sensitive to the base-emitter voltage and any change in temperature.

The emitter leg consists of a few inches of wire. I run current through it, creating a voltage drop.An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward. When you can control two motors to go either forward or backward — you can build yourself a robot!

A DC motor spins either backward or forward, depending on how you connect the plus and the minus. If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side.

And the motor will start spinning in one direction. If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction. Usually, you control the transistors from a microcontroller, such as Arduino. The most important thing is that all the transistors can handle enough current for the motor. Otherwise it will burn out. For example, if the motor draws 1 Ampere of current, you need transistors that can handle a minimum of 1 Ampere.

Because that turns into 3. I was trying to connect this to an Arduino, using its 5V supply, but failed because it was only 1V left for the motor!

A side-effect of how a motor works is that the motor will also generate electrical energy. When you disable the transistors to stop running the motor, this energy needs to be released on some way. If you add diodes in the reverse direction for the transistors, you give a path for the current to take to release this energy.

Without them, you risk that the voltage rises and damages your transistors. You can read more about this — and what to keep in mind if you want to use a PWM signal to control the speed of the motor, this article. The resistors going into each base is there to reduce the current to each transistor. Not sure how to calculate it? Have you built an H-bridge before?

Or do you have questions about the H-bridge? Let me know in the comments field below:. Skip to main content Skip to primary sidebar Skip to footer An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward. You normally use it with a microcontroller, such as an Arduino, to control motors. The H-Bridge circuit You can build an H-bridge with four transistors.

What Transistors To Use? The transistors you choose must: Handle enough current Use PNP or pmos at the top Have a low voltage drop between collector and emitter Current The most important thing is that all the transistors can handle enough current for the motor.

What turns the transistor on or off is the voltage difference between the base and the emitter. For example, you can use 3. That did not work.

TIP12x transistors give a 2V drop from the emitter to collector. Choose transistors with low voltage drop. Protection diodes and PWM mode A side-effect of how a motor works is that the motor will also generate electrical energy.Forums New posts Search forums.

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JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding. How to protect my mosfet h-bridge? Thread starter chris Start date Jan 5, The PWM signal is generated with a microprocessor.

While testing some code, the h-bridge mosfets blew. The only reason for the short that I can see is that in my code I accidently turned the both sides of the h-bridge on at the same time when changing directions - ie.

There really is no easy way to prevent what happened. You would have to have current sensing for each mosfet that will shutdown the gate during an over current event. Some mosfet drivers have over current fault protection built in, and use either a current sense resistor, or current sensing mosfets. The old rule is "the transistor is always faster than the fuse". You could add a transistor type current limiter in series with the power to the bridge such as this Current Limiting Circuit which would be fast enough.

This, of course, will add a small drop to the voltage due to the series current-sense resistor the current starts to limit at about a 0. Last edited: Jan 5, I have a lab power supply which has independently adjustable output voltage and output current. The output voltage can be set to say 12V, while the current limit trip point can be set to say 5A.

How to protect circuits from reversed voltage polarity!

MikeMl said:. Sceadwian Banned. There is unfortunatly no circuit capable of protecting itself from misuse. Everyone that experiments with electronics has done something like that, it's easier to simply tread carefully, considering if you have the knowledge to build a completly fool proof circuit in the first place you probably have enough knowledge to not make the mistake that requires it.

There could be a problem using the lab supply current limit to protect a transistor if the supply has a large output filter capacitor. The energy in the capacitance could be enough to blow the transistor even if the current limit is operational.

You have to create a delay circuit, either by using logic gates upon the output of your microcontroller, or program in a particular delay into your program. How much of a delay? Have to read the datasheets of your mosfets to see how long it takes to turn on and off.

I also have a.Welcome, Guest.

Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. Author Topic: Do you know a way to prevent short circuits in a h-bridge?

What is an H-Bridge?

Read times. Lewis Mojica Newbie Posts: 3 Country:. I'm designing a circuit to control a dc motor, now i'm working on the h-bridge and i made the following schematic: Then i realize that if both buttons are pressed at the same time it would produce a short circuit; every transistor would be in saturation so, vcc would be connected to ground from q1 to q4 and from q2 to q3.

Then i made this schematic: In the schematic above, the logics gates prevent the short circuit, so when both button are pressed the logics gates do not active any transistor. I also added diodes to each transistor to protect then from reverse voltage from the inductive load. Now, i'm wondering, are there a best way to protect this from circuit short circuit? Hey, thanks for reading my post. English is not my native language so, my apologies for any mistake on my writing.

Neilm Super Contributor Posts: Country:. HAve a look at H-bridge driver chips.

How to protect my mosfet h-bridge?

They include the bootstrap you will need to correctly drive the upper FETs I wouldn't use transistors and will include shorting and shoot through protection. Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe. Many H-Bridge drivers put a small dead time between high and low devices to eliminate cross coduction. Cyberdragon Super Contributor Posts: Country:. The problem with driver chips is that they are normally digital or PWM controlled and aren't meant for something this simple.

The logic chips would be far simpler than getting a driver chip to work. The type of transistors would be determined by the application. If it's a small battery-powered thing, bipolars would probably be fine. FETs would be suitable if the motor had significant load on it. The simplest solution, depending on the size of the device and power requirements, would just be two SPDT buttons or relays wired like a two-switch light circuit in a house.

So each button reverses the polarity of each side, and if you pushed both they'd just be at the same polarity again. Explodingus - someone who frequently causes accidental explosions. You cannot connect the bases of the high side transistors to the bases of the low-side transistors. They need separate drives, as their emitters are at different voltages.

Quote from: jmelson on March 29,pm.In the previous part of the series we went through the various circuits that can take logic level digital signals and make them suitable for driving the gates of the bridge power FETs. These circuits range from trivial to complex, and have some interesting properties, like turn-on and —off delay or limits on duty cycle that will have implications for our current discussion.

For both low- and high-side drivers, the two basic schemes are active low or active high control. With half-bridge drivers, which control one low and one high-side FET, the options are more complicated. Some simply expose two control signals active low or high each for the two controlled transistors. Again, these can be active low or active high. I will not bore you with all the possible combination check the datasheet for your part for the exact details but one example could be the following:.

Another control method is when the driver exposes only a single, three-state control input. Full-bridge drivers can expose an even wider variety of control options, since they drive four FETs. Sometimes they have four control signals for each individual FETs, or would look like two independent half-bridge circuits. When that happens like the HIPA from Intersilthe driver might start to limit the control patterns and drive modes you can use the bridge with.

This needs a minimum of 3 digital control signals, so if a full-bridge driver exposes less, you should expect limitations. Yet others the HIPA is a good example for that as well combine some analog functions to help implement closed-loop control. To simplify the discussion in the following chapter, I will assume that the bridge driver has four independent, active-high input signals, one for each FET. If your driver needs a different type of control, you can easily convert the signals to your particular needs.

H-Bridges – the Basics

One would think that we could simply drive them from the same input, through a simple inverter. Unfortunately in practice that would almost always be a bad idea but we will have to come back to that a bit later.

For the forward direction we can use the following table:. There are some other possible drive patterns that would result in one of these three drive modes. By carefully looking at the tables you can see some nice patterns emerge:.

Almost every bridge design needs to have a disabled, or power-down state. There are two possibilities to achieve that. One option is to have at least one of the motor terminals floating. The other is to short the motor terminals together. For lock anti-phase drive however a new state needs to be introduced for this purpose.

h bridge short circuit protection

In many cases, especially when a microcontroller is in charge of generating the signals for the MOSFETs, it is important that the default power-up state of the bridge is the power-down one. This way, the firmware running on the microcontroller is in no rush of initializing the bridge control signals.

It can work on getting the whole system up and running and only after all setup and initialization is done it needs to move the bridge out of power-down into the operational state. This can lead to the bridge turning on or worse, getting into a shoot-through condition during startup. To prevent this from happening, its good practice to include pull-up or pull-down resistors on each bridge control pin to make sure they are at a well-defined voltage at all times.

It is also good practice to set the resistors up such that they keep the bridge in its power-down state. If that happened, you would create a very low resistance path between your power supply and ground. There are several outcomes of such an experiment and none of them are pleasant.

At best, you have some sort of short-circuit protection that trips and your circuit simply looses power. If not, a lot of current will start flowing through your circuit. This current will start heating things up and eventually something will break. It will heat up the battery because of its internal resistance and overheated batteries can explode.An H bridge is an electronic circuit that switches the polarity of a voltage applied to a load.

These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. In particular, a bipolar stepper motor is almost invariably driven by a motor controller containing two H bridges. H bridges are available as integrated circuitsor can be built from discrete components. The term H bridge is derived from the typical graphical representation of such a circuit.

An H bridge is built with four switches solid-state or mechanical. When the switches S1 and S4 according to the first figure are closed and S2 and S3 are open a positive voltage will be applied across the motor.

By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor.

h bridge short circuit protection

Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4.

This condition is known as shoot-through. The following table summarises operation, with S1-S4 corresponding to the diagram above. One way to build an H bridge is to use an array of relays from a relay board.

A " double pole double throw " DPDT relay can generally achieve the same electrical functionality as an H bridge considering the usual function of the device. However a semiconductor-based H bridge would be preferable to the relay where a smaller physical size, high speed switching, or low driving voltage or low driving power is needed, or where the wearing out of mechanical parts is undesirable. Another option is to have a DPDT relay to set the direction of current flow and a transistor to enable the current flow.

This can extend the relay life, as the relay will be switched while the transistor is off and thereby there is no current flow. It also enables the use of PWM switching to control the current level. Alternatively, a switched-mode power supply DC—DC converter can be used to provide isolated 'floating' supplies to the gate drive circuitry. A multiple-output flyback converter is well-suited to this application.


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