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Abstract: A high-quality switching power supply efficiency is as high as 95%, and the switching power supply loss is mostly from switching devices (MOSFETs and diodes), so the correct measurement of switching device losses is critical for efficiency analysis. So how do we accurately measure switching losses?

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First, switching loss

 

Since the switch is a non-ideal device, its working process can be divided into four states, as shown in Figure 1. "On state" means that the switch tube is in the on state; "off state" means that the switch tube is in the off state; "on process" means that the switch tube is switched from off to on state; "off process" means that the switch tube is from conduction The conversion is turned off. In general, the main energy loss is reflected in the "on process" and "off process", a small part of the energy is reflected in the "on state", and the "off state" loss is very small, almost zero, negligible.

 

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Figure 1 Switching tube four state division

 

The actual measurement waveform is generally as shown below:

 

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Figure 2 Switch tube actual power loss test

 

Second, the conduction process loss

 

The energy consumed by the transistor switching circuit during the conversion process is usually very large, because the parasitic signal of the circuit prevents the device from switching immediately, and the voltage and current in this state are in an alternating state, so it is difficult to directly calculate the power consumption. The voltage and current are considered to be linear, so the loss can be roughly calculated by finding the area of the triangle, but this is not accurate enough. For digital oscilloscopes, advanced math functions are provided, so the loss of the conduction process can be calculated using the following formula.

 

1.png

 

Eon represents the loss energy of the conduction process

Pon represents the average power loss (active power) during the conduction process

Vds and Id represent instantaneous voltage and current, respectively

Ts indicates the switching period

T0, t1 indicate the start time and end time of the conduction process

 

Shutdown process loss

 


The closing process loss is the same as the conduction process loss calculation method, except that the start and end times of the integration are different.

 

2.png

 

Eoff represents the loss energy of the shutdown process

Poff represents the average power loss (active power) of the shutdown process

Vds and Id represent instantaneous voltage and current, respectively

Ts indicates the switching period

T2, t3 indicate the start time and end time of the shutdown process

 

Third, conduction loss

 

In the on state, the switch tube usually flows a large current, but the on-resistance of the switch tube is very small, usually in milliohms, so the energy loss in the on state is relatively small, but it cannot ignore. Using an oscilloscope to measure conduction loss is not recommended for voltage-to-current integration because the oscilloscope cannot accurately measure small voltages during turn-on. For example, when the switch is normally turned off, the voltage is 500V, and when it is turned on, it is 100mV. Suppose the accuracy of the oscilloscope is ±1‰ (this is a very bullish indicator), and the minimum measurement accuracy is ±500mV. It is impossible to accurately measure 100mV. It is even possible that the measured voltage is negative (100mV-500mV).

 

Since the small voltage at the time of conduction cannot be accurately measured, the energy loss error calculated by the method of integrating the voltage by the current is large. On the contrary, the current is large when turned on, so it can be measured accurately, so the current and on-resistance can be used to calculate the loss, as shown in the following formula:

3.png

 

Econd represents the loss energy of the conduction state

Pcond indicates the average power loss (active power) in the on state

Id represents the instantaneous current

Rds(on) indicates the on-resistance of the switch, which is given in the switch, as shown in Figure 3.

Ts indicates the switching period

T1, t2 indicate the start time and end time of the on state

 

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Figure 3 shows the relationship between on-resistance and current

Fourth, switching loss

 

Switching loss refers to the total energy loss, which consists of conduction process loss, shutdown process loss, and conduction loss, calculated using the following formula:

5.png

 

Five, switching loss analysis plug-in

 

High-end oscilloscopes usually also integrate switch loss analysis plug-ins. Because the on-state voltage measurement is not accurate, the calculation formula for the on-state can be modified. There are three main types:

 

● UI, U and I are measured values

● I2R, I is the measured value, R is the on-resistance, and the user inputs Rds(on)

● UceI, I is the measured value, and Uce is the voltage value input by the user to compensate for the problem of voltage and voltage uncertainty.

It is generally recommended to use the I2R formula. The figure below shows the switching loss test diagram of the ZDS4000 Plus.

 

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Figure 4 Switching loss test results

 

Summary

 

Switching loss testing is critical for device evaluation. With a professional power analysis plug-in, the power loss of the device can be evaluated quickly and efficiently, which is simpler and more convenient than manual analysis. For MOSFETs, the I2R conduction loss calculation formula is the best choice.

 

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