Diode has many uses. It could be as a rectifier, a reverse blocking device, a component in a switching converter or in a switching power supply and so on. Regardless, what application it is, they are the same parameters to consider. These parameters are all given in the diode datasheet. If you are looking for guidance on how to select a diode for design use, this article is for you.
Important Parameters to Consider on How to Select a Diode for Circuit Design
1. Forward voltage (VF)
The very first in the list on how to select a diode for circuit design is the forward voltage. This is the voltage drop measures across the anode and cathode when the diode is forward bias. This is given in the diode datasheet. In the datasheet, the forward voltage is defined in few tables and graph. Below is the forward voltage specification of a VS-E5TH3012S2L-M3 from Vishay Semiconductors.
The specification under primary characteristics is usually the highlight; like the best value the device can offer. The values under electrical specification on the other hand are more defined with respect to minimum, typical and maximum values.
On the graph (typical forward voltage drop characteristics) however, the relationship between the forward voltage and forward current is defined at specific temperatures. When you do the design, you must have idea what is the level of the current that flow to the diode. Use that current and project to the graph.
The graph is giving the most information. According to the graph, the forward voltage drop is maximum at negative temperature while minimum at the hottest temperature. Supposing the actual current flows to the diode is 10A while the expected temperature is only 25’C, then the forward voltage to use is 1.5V.
2. Forward Current (IF)
Another very important parameter to consider on how to select a diode is the forward current rating. This rating describes as the average rectified forward current in a diode datasheet. This is the maximum current the diode can handle. Above this, it will break. Below is the forward current rating of the VS-E5TH3012S2L-M3 diode.
3. Non-repetitive Peak Forward Current (IFSM)
Some datasheets may call this as non-repetitive surge current or peak current. This is the one-time surge current the diode can handle. This is given in a very specific condition like in below table. As long as the condition in the table is not exceeded, the diode will survive.
This diode rating is important when you are expecting a very high single pulse in-rush current during start-up of a system.
4. Repetitive Peak Forward Current (IFRM)
This is a repetitive current but not a DC or continuous current. There is a specific condition, like a type of waveform, duration and frequency and as long as it is not violated, the diode can survive.
This is useful information when you are expecting some pulses of in-rush current during start-up of any system.
5. Repetitive Peak Reverse Voltage (VRRM)
This is always in the front seat on how to select a diode for any applications. This is the maximum repetitive reverse voltage the diode can handle. Exceeding this value will immediately killing a diode. Below specification is from the datasheet of VS-E5TH3012S2L-M3 diode.
This rating is very important because when the diode is reverse biases, it will see the power supply voltage. Furthermore, in inductive circuit, there is a kickback voltage that will add up to the power supply voltage making the circuit open circuit voltage to be very high.
6. Power Dissipation
Power dissipation is equally important thing to consider on how to select a diode. The forward voltage (VF) and the forward current (IF) will result to certain power dissipation. When a diode is used in a switching converter, there is a switching loss that will add to the conduction loss (VF time IF). The total power dissipation must not exceed the diode capability. Some datasheet provide typical power dissipation while others are not. Typical power dissipation is the diode power dissipation rating taken at nominal ambient temperature which is usually at 25’C.
The diode power dissipation capability is dependent to the junction temperature, application temperature (ambient or case temperature) and the thermal resistance. It can be computed using the equation
Power dissipation, capability = (TJ max – Tamb max) / RthJA
Power dissipation, capability = (TJ max – Tc max) / RthJC
Where TJ max is the maximum junction temperature given in the datasheet, Tamb max is the maximum ambient temperature of operation, Tc max is the maximum case temperature and RthJA is the thermal resistance from junction to ambient which is also provided in the datasheet.
7. Junction and Storage Temperature
A diode junction temperature is often provided in a range like in below table. Outside this temperature range, the diode will break. On the other hand, there is also a storage temperature specification. Storage temperature is a non-functional rating that means the diode is non-conducting like it is just in the storage bin or warehouses. However, this is still critical to follow, otherwise, the diode will fail.
8. Thermal Resistance
The thermal resistance could be from junction to case (RthJC) or from junction to ambient (RthJA). This is very important diode rating in order to compute the diode power dissipation capability with respect to a certain case or ambient temperature. This rating is specified in either K/W or ‘C/W. Don’t be get confuse as they are same. Refer to the Power Dissipation section in this article on how to use thermal resistance.
9. Reverse Recovery Time (trr)
When the diode is used in a switching converter, switch mode power supply or any circuit that changes from forward and reverse bias continuously, reverse recovery time is an important parameter. In such applications, the reverse recovery time desired is very low. The longer the reverse recovery time, the longer the reverse current will flow and this corresponds to a switching loss.
10. Current Stress
Current stress is a measurement of how big the actual current allowed to flow to the diode. A conservative current stress level is in the range of 50%-70%. However, in some cases, 80%-90% stress is allowed. This is all depends to the circuit designer and the constraints that he is facing.
Current Stress = (Actual Current / Current Rating) x 100%
Actual current is the current that flow to the diode while current rating is could be the forward current or the peak surge current of the diode.
How to Derive the Actual Current
If you have the circuit running, you can use an ammeter and connect it in series to the diode to get the actual current. Keep in mind to ensure the ammeter fuse can handle the current level. If you have and oscilloscope, use it with a current probe. Clamp the current probe to the wire that connects to the diode. You can set the measuring equipment to either record the average (DC) or the rms value. For oscilloscope, both parameters could be displayed at the same time. The peak surge current cannot be measured using an ammeter. The most appropriate equipment is an oscilloscope.
11. Voltage Stress
The same with the current stress, a voltage stress is a measurement on how big the actual voltage is allowed to the diode. The conservative level is around 50%-70% but in some cases and constraints it will go as high as 90%. I leave this to the circuit designer.
Voltage Stress = (Actual Voltage / Voltage Rating) x 100%
Actual voltage is the voltage that the diode will experience during reverse bias while voltage rating is the repetitive peak reverse voltage.
How to Get the Actual Voltage
The actual circuit peak reverse voltage can be measured using a voltmeter. Just connect the voltmeter across the diode. Connect the voltmeter negative probe to the anode. On the other hand, connect the voltmeter positive probe to the diode cathode. If the diode behavior is continuously turn-on and turn-off, use a oscilloscope to measure the voltage. Follow the same probe connections.
12. Power Stress
Power stress is an indicator how big the power dissipation is allowed to the diode. In most cases (a conservative way), power stress is set to 50%. But in some design limitations and constraints, it will reach as high as 80% and beyond.
Power Stress = (Actual Power / Power Rating) x 100%
Actual power is the actual computed power dissipation by the diode. On the other hand, power rating is the equivalent power dissipation capability of the diode. This is discussed in section Power Dissipation above.
13. Thermal Stress
Thermal stress is an indication use to assess how high the temperature the diode is allowed to operate. Conservative value is around 50%. However, in some design constraints, it will reach to 80% to even slightly higher. There is no issue as long as the design engineer is able to conclude that the diode will not fail until the expected operational life of the product.
Thermal Stress = (Actual Temperature / Temperature Rating) x 100%
Actual temperature is could be case or junction temperature while temperature rating is the temperature specified in the datasheet.