Today I will introduce to you how to choose a diode in PCB design? The main aspects are as follows:
- 1. Forward voltage
- 2. Forward current
- 3、非重复峰值正向电流 (IFSM)
- 4. Repeat Peak Forward Current (IFRM)
- 5. Repeat Peak Reverse Voltage (VRRM)
- 6. Power consumption
- 7. Junction temperature and storage temperature
- 8. Thermal resistance
- 9. Reverse Recovery Time (TRR)
- 10. Current stress
- 11. Voltage stress
- 12. Power stress
- 13. Thermal stress
Diodes have many uses and can be used as rectifiers, reverse blockers, switching converters, etc. But the same parameters need to be taken into account regardless of where they are used, and in general, they can be found in the Datasheet. Important parameters to consider when selecting a diode for circuit design.
diode
1. Diode forward voltage (VF)
Forward voltage: is the measured voltage drop between the anode and cathode when the diode is biased. In the Datasheet, the forward voltage is written in the following tables and graphs. Below are the forward voltage specifications for the VS-E5TH3012S2L-M3.
Diode forward voltage
In the case of electrical characteristics, minimum, typical, maximum values are usually provided.
In a typical forward voltage drop characteristic, the relationship between the forward voltage and the forward current is at a specific temperature. When designing, you need to know what the current level of the diode is, use that current and project it on a graph.
A lot of information is provided in this chart. The forward voltage drop is maximum at negative temperatures, but smallest at hottest temperatures. Assuming the actual current flowing to the diode is 10A and the expected temperature is only 25°C, the forward voltage to be used is 1.5V.
2、二极管正向电流 (IF)
The forward current rating is the average rectified forward current in the diode data, which is the maximum current that the diode can handle. If it is greater than this value, the diode may be damaged. Below are the forward current ratings for the VS-E5TH3012S2L-M3 diodes.
Diode Forward Current (IF)
3. Non-repetitive Peak Forward Current (IFSM)
Also referred to as non-repetitive inrush current or peak current in some datasheets, it is a one-time inrush current that a diode can handle. As shown in the figure below, the diode will not be damaged as long as the conditions in the table are not exceeded.
非重复峰值正向电流 (IFSM)
When there can be very high single-pulse inrush currents during system start-up, diode ratings are very important.
4. Repeat Peak Forward Current (IFRM)
Here is a repetitive current, but not a DC or continuous current, with a specific condition, such as the type, duration, and frequency of the waveform, as long as it does not exceed this value, the diode can be allowed.
Repeat Peak Forward Current (IFRM)
When some inrush current pulses are expected during system start-up.
5. Repeat Peak Reverse Voltage (VRRM)
This is an important factor in the selection of a diode, the repeat peak reverse voltage is the maximum repeat reverse voltage that the diode can withstand, and more than this value will damage the diode. Below is the datasheet from the VS-E5TH3012S2L-M3 diode.
Repeat Peak Reverse Voltage (VRRM)
This rating is very important because when the diode is back-biased, you see the voltage supply, and in addition, in the inductor circuit, there is a backlash voltage that is added to the supply voltage, which causes the circuit to have a very high open-circuit voltage.
6. Power consumption
Power consumption is also a very important factor when choosing a diode. Forward voltage (VF) and forward current (IF) result in a certain amount of power dissipation. When switching the conversion diode, the switching loss increases the transfer loss (VF time IF).
Some datasheets will provide the typical power consumption of a diode, while others will not. Typical power consumption is typically a diode power rating obtained at a standard ambient temperature of 25°C.
The power dissipation capability of a diode depends on the junction temperature, application temperature (ambient or case temperature), and thermal resistance. It can be calculated using equations
功耗，能力 = (T J max – Tamb max) / Rth JA
or
功耗，能力 = (T J max – Tc max) / Rth JC
- TJ max is the maximum junction temperature given in the datasheet
- Tamb max is the maximum operating ambient temperature
- Tc max is the maximum case temperature
- Rth JA is the thermal resistance of the junction to the environment
7. Junction temperature and storage temperature
The diode junction temperature is usually within the range of the table below, and once this range is exceeded, the diode will be damaged. On the other hand, there is also the storage temperature. The storage temperature is a non-functional rating, meaning that the diode is not conductive. It looks simple, but it's also very important, and if not stored properly, it can also cause the diode to fail.
Junction temperature and storage temperature
8. Thermal resistance
The thermal resistance can be from the junction to the shell ((Rth JC) or from the junction to the environment (Rth JA). This is a very important diode rating. It is mainly used to calculate the power dissipation capability of the diode under specific conditions or at ambient temperature. How to use thermal resistance is discussed in the above section on power consumption.
Thermal resistance
9. Reverse Recovery Time (TRR)
When a diode is used in a switching converter, switch-mode current, or any forward and reverse bias continuously varying circuit, the reverse recovery time is a very important parameter. In such applications, the required reverse recovery time is very short. The longer the reverse recovery time, the longer the reverse current will flow, which corresponds to the switching loss.
Reverse Recovery Time (TRR)
10. Current stress
Current stress is a measure of how much actual current is allowed to flow to a diode. In general, the current stress level is between 50% and 70%. In some cases, 80%-90% will be allowed, depending on the design of the circuit.
Current Stress = (Actual Current / Rated Current) x 100%
The actual current is the current flowing to the diode, while the rated current can be either the forward current or the peak inrush current of the diode.
Diode physical diagram
How to get the actual current
If the circuit is running, you can use an ammeter to connect it in series to a diode to get the actual current. This is to make sure that the ammeter fuse is able to handle the current level. If an oscilloscope is available, it can be used with a current probe.
By clamping the current probe onto the wire to which the diode is attached, the measurement device can be set to record average (DC) or rms values. For oscilloscopes, these 2 parameters can be displayed at the same time.
11. Voltage stress
Like current stress, voltage stress is a measurement of how much actual voltage a diode allows. Generally around 50%-70%, but in some cases and limits, it can be as high as 90%.
Voltage Stress = (Actual Voltage / Rated Voltage) x 100%
The actual voltage is the voltage that the diode will experience during reverse bias, while the rated voltage is the repeating peak reverse voltage.
How to measure voltage stress
The actual circuit peak reverse voltage can be measured using a voltmeter, just connect the voltmeter at the 2nd end of the diode, connect the negative probe of the voltmeter to the anode, and connect the positive probe of the voltmeter to the cathode of the diode.
If the diode is continuously turned on and off, an oscilloscope is required to measure the voltage.
Diode physical diagram
12. Power stress
Power stress is the allowable power dissipation of a diode. In most cases, the power supply stress is set to 50%. But in some designs, it can be as high as 80% or more.
Power Stress = (Actual Power / Rated Power) x 100%
Actual power is the actual calculated power consumption of the diode. On the other hand, power rating is the equivalent power dissipation capability of the diode.
13. Thermal stress
Thermal stress is used to evaluate how high the temperature of the diode is operating, generally around 50%. In some designs, it may reach 80%. As long as the engineer can conclude that the diode will not fail before the expected useful life of the product, there is no problem.
Thermal Stress = (Actual Temperature / Rated Temperature) x 100%
The actual temperature may be the enclosure or junction temperature, while the rated temperature is the temperature specified in the datasheet.
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