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PDM Installation & Configuration: Half Bridges & Full Bridges

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Half Bridges & Full Bridges


00:00 - When you're specifying a PMU for your project, a couple of terms that you'll see quite often are half bridge and full bridge.
00:07 These terms are very important to the specification and operation of a PMU so we'll have a look at them in detail now.
00:13 To control the delivery of power to a part of the electrical system in our project, we can either wire that component to the electrical power source in the vehicle, not only the battery and switch it on the ground side, or wire the component to ground and switch the battery connection.
00:32 This is known as low side or high side switching respectively.
00:36 Most commonly, the outputs on the PMU are high side switches, meaning when they're turned on, they provide a connection to the electrical power source in the vehicle.
00:45 And it's expected that the connected load will be wired to the vehicle ground on its other side.
00:51 When the PMU outputs are turned off, they're essentially connected to nothing at all and are in a state called high impedance.
00:58 A half bridge is a particular arrangement of two MOSFET switches connected to a single PMU output that allows it to both high side and low side switch.
01:09 When we command a half bridge output to be high, it's connected to battery power and when we command it to be low it's connected to the vehicle ground.
01:18 There's no longer a high impedance state where the output isn't connected to anything at all.
01:23 What's important to understand is that there's never a state where both of these switches are closed at the same time as that would directly connect the battery power to ground via the MOSFETs inside the PMU and destroy the output circuitry.
01:37 This initially seems like it would give us the same result as a high side switch because if we wire the connected load to the half bridge output on one side and ground on the other, and then switch the half bridge to high, it will connect the battery to the load, switching it on.
01:52 If we command the half bridge output low, we'll have a ground connection on both sides of our load, so it'll be switched off.
01:59 What it does gain us though is an amount of flexibility on how we wire our electrical system as we can now wire battery power directly to one side of our load and then command the half bridge output low to turn the load on.
02:11 Commanding this output high in this circumstance means that we'll have battery power connected to both sides of our load so there's no voltage difference across it, no current will flow and it'll be off.
02:22 There's another effect the half bridge switch causes being specific to connecting electromotive loads.
02:28 It can give a braking effect, rapidly slowing a connected DC motor down when it's switched off but we'll discuss this in a little more detail in a later course module because it's very important to some of the integrated body control functions in many PMUs.
02:41 With an understanding of how a half bridge works we can now look at how a full bridge works.
02:47 Also this is commonly called an H bridge.
02:50 A full bridge is simply two half bridges, the control logic of which is linked together inside the PMU.
02:58 This means that a full bridge will have two physical output pins on the PMU, one for each half bridge.
03:06 Commonly referred to as output A and output B.
03:09 To connect a load to a full bridge, it's wired between these two output pins.
03:16 The internal linking of the control logic is such that when one half bridge is commanded to switch high, the other is automatically commanded to switch low.
03:25 So as an example, if I have a DC motor connected to a full bridge, one of its pins will be connected to output A and the other to output B.
03:34 If I command the full bridge high, output A is connected to the battery power and output B is connected to ground.
03:41 Current will flow through the DC motor causing it to spin.
03:45 If I then command the full bridge low, output A will be connected to ground and output B will be connected to the battery power.
03:53 This reverses the direction of flow through the DC motor, causing it to spin in the opposite direction.
03:59 There are cases in the automotive world where we might want to have a motor spin in opposite directions at different times such as extending a wing to get more downforce but then retracting it again when that downforce is no longer required and the associated drag is undesirable.
04:13 Or a more mundane example, an electric window motor where we want to be able to both lower and raise the window.
04:19 Once again, we can command other modes of full bridges where we connect both sides of the connected load to either battery power or the vehicle ground to generate a braking effect but again this is something we'll discuss in detail in a later module.
04:34 In this module, we've talked about two common terms you'll hear when specifying a PMU for your project, being half bridges and full bridges.
04:41 Half bridges allow an output to be both high side and low side switching, increasing the flexibility of our system.
04:48 A full bridge is simply two half bridges with their internal control logic linked together.
04:53 We can use a full bridge to supply power to the connected load in opposite directions at different times which can be required for some DC motor control applications.

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