IGBT has two types of losses. One is conduction and the other one is switching. Conduction loss is due to the voltage drop between collector and emitter times the collector current. On the other hand, switching losses is due to the dynamic parameters of the IGBT. If you are looking for an explanation on how to derive IGBT power losses, this article is for you. Keep reading.
How to Derive IGBT Power Losses – Step by Step Guide
A. Compute the Conduction Loss
First to discuss here on how to derive IGBT power losses is the conduction loss. Conduction is also called a static loss. This will occur when the IGBT channel is conducting and there current flow.
1. From the IGBT datasheet, get the VCEsat value. This is the collector-emitter saturation voltage. If you are aiming for a worst case loss, use the maximum VCEsat. On the othee hand, if your objective is to compute the system efficiency, you may consider the typical value. Below is an example datasheet declaration of the VCEsat.
2. Compute the actual application collector current. You need to understand your design and determine the typical collector current or the maximum collector current. Typical value is for efficiency purposes since you need a near accurate value for it. On the other hand the maximum value is to consider the worst case to provide more design margin.
3. To compute the conduction loss, simply multiply the VCEsat to the actual application collector current
Ploss Conduction = VCEsat x Collector Current
B. Compute the Switching Losses
The next type of loss that we are going to discuss is switching loss. Switching loss occurs during dynamic operations, like turn-on and turn-off and general due to continuous switching action. In IGBT, there is a loss due to the gate charge, due to the rise and fall time, and there is also a loss due to the turn on and turn off energy. Other engineers may have other method on how to derive IGBT power losses; I am only sharing here what I usually do.
Switching Loss due to Gate Charge
1. Get the gate charge from the IGBT datasheet. The gate charge could be specified like below
2. Know the voltage that driving the gate-emitter of the IGBT. It is a design given such as 12V, 15V, 18V and so on.
3. Know the switching frequency. This is also a design given such as 10kHz, 15kHz, 30kHz and so on.
4. Compute the power loss due to gate charge using the equation
Power loss gate charge = ½ x Qtotal x V x Fsw
Qtotal is the total gate charge specified in the MOSFET datasheet, V is the applied VGS (gate to source voltage), Fsw is the switching frequency
Switching Loss due to Turn on and Turn off Energy
1. From the IGBT datasheet, look for a graph of gate resistor (Rg) versus energy (E). It will look like below.
2. From the graph, locate the curve for the turn on. It is Eon on the above graph. In the above graph, there are two Eon. One is 125’C and the other one is for the 150’C. You may use either temperature, which you think will most likely to occur. Go to your actual circuit and use the gate resistor value and project that value in the curve. For instance, the gate resistor value is 4 ohms, from the above graph at Eon = 150’C, the energy is around 15mJ. To get the turn on loss, use the equation
Ploss Eon = Eon X Frequency
Frequency is the converter switching frequency and it is a design given. Take note that the graph is specified at 100A collector current which is the maximum capability of the IGBT. If the actual application collector current is less than 100A, the loss is lower.
3. Again from the graph, repeat item 2 above but this time for the Eoff. Then compute the turn off loss using below equation
Ploss Eoff = Eoff X Frequency
Switching Loss due to Rise and Fall Times
1. Look for the rise time and fall time specifications in the datasheet. It could be defined as below table.
2. Use below equation to compute for the loss due to rise and fall times.
Power loss rise and fall times = ½ x (trise + tfall) x Irms x V x Fsw
trise is the rise time specified in the IGBT datasheet, tfall is the fall time specified in the IGBT datasheet, Irms is the rms value of the collector current of the IGBT (actual application), V is the collector to emitter voltage and Fsw is the switching frequency
The resulting IGBT Power Losses would be
The total power loss of an IGBT is the sum of the conduction loss, gate charge loss, Eon and Eoff loss and rise and fall time related losses.
Power Loss total = Ploss Conduction + Power loss gate charge + Power loss Eon + Power loss Eoff + Power loss rise and fall times
Summary – How to Derive IGBT Power Losses
1. IGBT power losses are due to conduction and switching losses
2. Conduction loss is a static loss due to the channel conduction and the collector current
3. Switching losses is could be the sum of the Eon and Eoff losses, gate charge loss and rise and fall time loss
4. Eon and Eoff losses are dependent to the gate resistor. This is the externals gate resistor.
Above technique is not the universally adapted by all engineers. However, it is more than enough to give you idea how big the power losses of your design. With this, you can size your IGBT correctly and avoid failure. If you have a better way on how to derive IGBT power losses, kindly share it here.