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PDM Installation & Configuration: PWM Control, Soft Start & Motor Braking

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PWM Control, Soft Start & Motor Braking


00:00 - We've established in earlier course modules that a PMU replaces the relays in a conventional automotive electrical system with solid state switching elements called MOSFETs.
00:10 This moves us away from having mechanical movement required to make and break the circuits which gives us a great increase in reliability but also means the switching is much much faster.
00:22 This faster switching allows us to use a technique called pulse width modulation or PWM to not just switch a connected device on and off, but actually adjust the amount of power that we deliver to that device.
00:36 We've already covered PWM in our wiring fundamentals course and we'll just refresh it quickly here to make sure that we're all on the same page.
00:42 When we PWM control a device, we switch the power to it on and off very quickly.
00:49 If we do this fast enough, the connected device won't see each individual on and off switching event and will instead just be delivered a lower overall amount of power.
00:59 The speed at which we switch the signal on and off to the device is called the PWM base frequency.
01:05 If this is for example say 1000 Hz, this means we switch the power on and off for the device 1000 times a second.
01:14 So this is the P part of PWM, we have 1000 pulses being delivered to the device every second.
01:22 The W part of PWM is the width of these pulses, meaning how much of that pulse is on and how much is off.
01:29 The M, the modulation part just means that we can change the width of that pulse to modulate how much power we're actually delivering to the connected device.
01:38 If we have a pulse width of 0%, this means that for each pulse of power delivered to the device, it is on for 0% and off for 100% so no power would actually be delivered.
01:51 Conversely, if we have a pulse width of 100%, each pulse of power to the device is on for 100% and off for 0% so we would have the maximum possible amount of power delivered to the device.
02:04 We can vary this percentage anywhere in between to deliver varying amounts of power.
02:08 This pulse width percentage is called the duty cycle.
02:12 This sounds amazing right? Unfortunately there is a downside to PWM switching though and that is heat generation.
02:19 Every time the MOSFET switches on and off it has to dissipate a small amount of power which manifests as heat within the MOSFET itself.
02:28 If we're PWM controlling a connector device at 1 KHz, that means the MOSFET is getting a little injection of heat 1000 times a second.
02:37 The amount of heat is related to the current passing through the MOSFET when it's in the on portion of the pulse and the higher the current, the more heat generated.
02:46 If this current is high enough and the PWM base frequency is high enough, the generated heat can quick destroy a MOSFET.
02:54 PWM manufacturers are of course aware of this and will have limitations in place on the PWM base frequency and power delivery to attach to devices.
03:03 They'll also have internal thermal monitoring systems to make sure that nothing's getting too hot.
03:08 It's a common misconception when installing a PMU that every output will be able to be PWM controlled but this is very seldom the case so make sure that you read the manufacturer's documentation to determine if they offer PWM control.
03:24 PWM is often used in a specific way, known as soft starting.
03:28 This is for a connected electromotive load such as a fuel pump or a radiator cooling fan.
03:33 Soft starting this load means that we pulse width control the connected output, only when the device is first turned on.
03:41 The PWM duty cycle is then ramped up from 0% to 100% over a set time period.
03:49 This gradual increase in the delivered power to the connected load avoids a large in rush current and can lead to increased reliability as it's gentler to the connected loads, accelerating them up to their full speed in a more controlled manner than just applying full power from a dead stop.
04:05 Although soft starting uses PWM control, there are PMUs on the market that feature soft starting but not user configurable PWM control.
04:15 This is because the PWM control period during soft starting generates heat in the MOSFET but only for that soft start period which is relatively short and after it's finished, the MOSFET can cool down again.
04:27 Once again, make sure to read the documentation for your PMU thoroughly to determine if it has soft start features.
04:33 Another strategy encountered when controlling electromotive loads, motors in particular is braking.
04:39 This is essentially the opposite of soft starting and instead has the aim of bringing a running motor to a physical stop as fast as possible.
04:48 When a motor load is running, there's some energy stored in the magnetic field and when power is disconnected, this energy is then released back into the system and can cause the motor to run on for an unexpected amount of time.
05:01 If when power is removed from the motor, its two pins are then both connected to ground or both connected to power, this energy dissipates quickly and the motor will come to a halt very suddenly.
05:12 A good example of this is a common battery powered electric drill.
05:16 You'll notice that when the drill is running at full speed and you release the trigger, the chuck comes to a stop almost instantly.
05:23 This is because the control electronics inside that drill are actively braking that motor by shorting its terminals together.
05:30 There are a couple of common use cases for motor braking in the automotive world and they are with window wiper motors and pop up headlight motors.
05:37 When these systems reach specific points, the motors connected to them need to be actively braked to avoid any overrun.
05:45 You can imagine that if every time you turned off your windscreen motors, they moved back down to their park position on the screen but then the motor overran and they came halfway back up, that would be slightly infuriating.
05:56 Most PMUs have specific logic and output pins that allow motor braking in these situations.
06:02 In this course module, we're revisited the basics of PWM control and discussed how it can be used to adjust the amount of power delivered to a connected load instead of just being all off or all on.
06:14 PWM control has a downside of generating heat within the PMU and is not usually available on every output.
06:22 Soft starting is a particular form of PWM control used to gently ramp up the amount of delivered power to a device on start up only and motor braking is a technique used to quickly stop a connected motor so that it does not overrun an intended stop position.

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