# Tag Info

70

Probably you already have it, and just didn't know it. If you are driving a motor with a half-bridge or H-bridge and PWM or similar, you have regenerative braking. Let's consider a half-bridge, since for this analysis we will run the motor in only one direction: First, let's consider non-regenerative braking. If the bridge output is high (S1 closed, S2 open)...

42

In short: You have linear control of the 'speed' by applying a pwm signal, now the frequency of that signal has to be high enough so that your DC Motor only passes the DC component of the PWM signal, which is just the average. Think of the motor as a low pass filter. If you look the transfer function or relationship angular speed to voltage, this is what ...

19

For low voltages, it seems like the DRV8837 is pretty good: - With an 800mA load, the volt drop is: - $I_O\cdot R_{OS(ON)}$ = 800mA x 0.33 ohms = 0.264 volts. At this current, the power dissipation will be 0.8 x 0.8 x 0.33 watts = 211 mW. Compare this with the L293 power dissipation at about 800mA - maybe about 3V is lost giving rise to a power ...

19

Not a ground loop. But a short circuit: Always use fuses with batteries. Instead, you can make it like this, so that there is only one common ground: Drawback is unbalanced load on the battery. Meaning the left battery depletes faster, possible damaging it when the set is deep discharged. I recommend a battery balancer, or a 24V-12V power supply instead.

16

Each pin on an Arduino can handle 40 milli amperes, not 40 Amperes. That too in ideal conditions (temperature, Vcc), actual allowable current can be a fair bit lower. Besides current limitation of the Arduino pins, a key failure condition is the LDO voltage regulator on the Arduino board, which will overheat and can get destroyed if it is made to source high ...

16

The PWM frequency supplied to a (presumably) brushed DC motor needs to be high enough that the combination of mechanical inertia and inductance of the coils is sufficient to smooth out the mechanical impulses of each pulse. This minimum would differ from motor to motor. Too low a frequency, and the motor motion will be perceived as a series of jerks, or a ...

15

The triangle is a symbol for a buffer, driver or amplifier, whatever you call it. The transistors drawn inside the triangle are there to let you know what is the type of your output. Thanks to this drawing you can see that the outputs are switched via BJTs between the power supply rails using some kind of Push-Pull topology. Knowledge about the type of ...

15

Flogging the FREDs Voltage fed converters with transformer isolation will exhibit ringing in the secondary. Ringing is caused by parasitic inductances and capacitances in the circuit, with the dominant elements will being the transformer leakage inductance ($L_ {\text {Lk}}$) and junction capacitance ( $C_j$)of the bridge diodes. The diode data ...

15

Recently, I bought the H-Bridge VNH7070BAS from ST because it would drive up to 15A It will not drive anything like this level of current for anything more than a few micro seconds before the automatic current limit circuit operated. The internal transistors together contribute circa 0.1 ohm impedance in the current path and, at (say) 10 amps continuous, ...

12

I think you will encounter two challenges in designing this. The first is getting the high side to turn on. The source of the high-side moves up and down relative to ground, so a voltage source referenced to ground won't be able to drive it without exceeding the maximum gate-to-source voltage $V_{GS}$. For RFG50N06, this is 20V. The second is that you ...

12

Try probing on the power supply rail. I bet you see those spikes on there. It'll be due to the lead length between your bench supply and the MOSFETs. Clearly you won't see it on the lower FET side because your scope is referenced to that rail but, if you probed back at the power supply I bet you would. Try a 1uF or 10uF ceramic across the power rails close ...

12

The diodes serve two distinct purposes. Under regenerative braking, they return the generated voltage to the power supply (where with suitable electronics, it can be used to recharge the battery). Note that unless the motor is being run above its normal speed, the generated voltage will be no more than the supply voltage, so it is within the voltage rating ...

11

Your first concern in selecting a gate driver is to find one that can drive enough current to switch your selected MOSFETs fast enough for your application. As a rough estimation, you can divide the total gate charge of your MOSFET by the current the driver can sink/supply. $$t_{on}=\frac{Q_g}{I_g}$$ Using the worst-case values for IRF1405 and the slower ...

11

At minimum, you need to use a frequency so that the motor "sees" the average and doesn't react to individual pulses. That is usually a few 100 Hz. However, there are other effects that the motor doesn't care about but you might. Individual sections of wire in the windings may vibrate slightly with the PWM frequency, which causes audible whine. This is ...

11

How are you decoupling the 12V supply? One possible failure mode is that inductive spikes from switching off the motor current (i.e. at the PWM rate) are dumped into the 12V supply via the flyback diodes. Yes, that's supposed to happen, but... If the 12V supply is not decoupled, and is sourced from a PSU not a rechargeable battery, or is sourced via a long ...

11

Your motor is likely geared down, because 150 rpm is only 2.5 revolutions per second. At 50 rpm, your motor will require more than a second to perform one revolution. That having been said, the switches in your h-bridge don't dissipate much power when they are on (essentially zero volts) or when they are off (zero current). They only have both voltage and ...

10

A TEC is polarized in the sense that how it is connected matters. If you want to be able to heat and cool an element then a full Hbridge will work. This will allow you to pump current both directions across a TEC's terminals. If you apply a positive voltage to a TEC in one polarization then side A will get warm and side B will cool. If you then reverse ...

10

I first saw the following circuit in a TI app note many years ago. It works well and is robust. I have used it or variations of it in several projects (and products). simulate this circuit – Schematic created using CircuitLab There are a couple of things to consider: 1) This circuit inverts the drive signal. 2) R3 is added to ensure the FET drive ...

10

To use this circuit with a 12V supply R5 & R6 must be rated for at least 0.25W power and the 100 ohm resistors must be rated for at least 2W power. Depending on the transistors used these 4 resistors (100 ohm) may need recalculation to avoid excessive base current and power dissipation. However this circuit will not work. One problem is that it ...

10

If the motor is producing power, the net power into the motor must be positive, so the net current out of the batteries must be in the direction that drains them, so you are fine. If the motor is being regeneratively braked, then power can flow out of the motor and can push the supply voltage up and charge the batteries (this is used to advantage in ...

10

On your layout the decoupling cap (finger-painted red) is very far away from the chip, there is a 5R resistor in series (purple) and the current loop area (highlighted in yellow) is quite large. This large loop area adds inductance to the power supply impedance as seen from the chip. Wet finger in the wind, about 10-20 nH. With a di/dt around 1A per 1-2ns ...

9

This is a classic snubbering problem. A diode can't instantaneously go from conduction to blocking; the charge in the PN junction needs to get swept out, and an RC snubber across each diode should help this. I used to design industrial soft starters and on the medium-voltage units we had a lot of design work around this particular aspect. It's been a long ...

9

The reasons are different depending on if it is a BJT or a MOSFET. The effect, though, is the same - it's there to reduce current and protect the IO pins on the controller that are driving the transistors. On the BJT the base -> emitter junction is essentially like a diode, and without a resistor would be like a near short circuit. The resistor stops the ...

9

P-channel MOSFETs tend to have a higher Rds(on), making them less efficient (for the same price). For a small H-bridge, the simplicity of using them makes them practical However, for high power applications, the extra complexity of driving N channel FETs can be justified by improved efficiency from lower Rds(on).

9

Your P channel MOSFETs are connected upside down: - Notice the little diode symbols inside the MOSFETs? They will permanently conduct current and the MOSFETs will not switch off.

9

Those old bipolar processes could only be used to make crummy lateral PNP transistors with quite low gain so not very good for high current. Using all NPN, especially those NPN, transistors is bad (because of the extra Vbe drop), but not as bad as it would be with monolithic PNPs. Popular? Probably for legacy applications and hobbyists, but I doubt any ...

8

Can it be done? Yes Has it been done? Yes Will it do as expected? half the switching losses? Yes & if care was taken over the right-leg device selection trading speed for conduction losses then you could further improve the powerCore losses. Quick model with some REALLY badly optimised output filter & not really tuned, just to prove a point &...

8

A practical motor behaves roughly like a resistor and inductor in series with a real motor. For efficient operation you need should switch between connecting the motor to the supply and shorting it out. While the motor is connected to the supply, the current will become more positive. When shorted, it will become more negative. Efficiency will go ...

8

Your o-scope time base is set to 1.00 seconds and, by the looks of it your control signals will be switching at 0.5 Hz. This is waaaaaaaaaaay too low in frequency. You have (in your driver circuit) bootstrap components that help give good drive signals to your upper N channel MOSFETs but, these bootstrap circuits can only work when you have sufficient drive ...

7

That is just a notation, not by any means a schematic. The "triangles" themselves symbolise high current drivers, the two "transistors" drawn there are just to symbolise what the circuit is doing. There more components "inside the triangle", that for sake of brevity are not displayed. This is similar to pseudocode and real source code in a given ...

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