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This series uses reference circuits for evaluation boards for converter ICs to explain important characteristics in selecting various discrete components.
When explaining the characteristics, use LTspice(R) for each characteristic, change the constants of the parts or the parts themselves, check the changes on the circuit with simulation waveforms and calculated values, and check the relationship between the characteristics and the circuit. I will explain.
This time, we will explain how to select the catch diode required for the asynchronous rectification type buck converter circuit, using LTspice simulations to check the effects of diode characteristics.
Also, please refer to the following for LTspice and evaluation kits used in the explanation.
[How to download / use LTspice]
・ LTspice download page (link to Analog Devices website)
*If you want to know how to use LTspice, please check the Company article below.
[Evaluation board used/mounted regulator/board purchase information]
・Evaluation board: DC1832A (link to Analog Devices website)
・ LT3995 Asynchronous buck regulator (link to Analog Devices website)
・Evaluation board DC1832A is available at Macnica-Mouser.jp. (Link to Macnica-Mouser.jp)
(Note)
This article does not describe how to select peripheral components (diodes) for the above converter IC LT3995. It is used as a sample circuit example to check the characteristics of the diode on LTspice.
table of contents
・ The role of the diode and its effect on the converter IC
1. Role of Diodes and Effects on Converter ICs
The role of the diode in the asynchronous rectification buck converter circuit is to perform rectification operation to supply current to the output side while the high-side switch in the converter IC is OFF. In other words, it acts as a low side switch.
At that time, the diode's forward voltage (hereafter referred to as VF) becomes a loss and affects the efficiency of the power supply circuit.
Using the LT3995 circuit on the DC1832A as an example, the role of the diode is shown below.
Losses also occur during switching when the diode is turned on and off. The time it takes for a diode to turn from ON to completely OFF is called reverse recovery time (trr). After turning off, the current flows in the opposite direction by a certain amount. The greater the current, the greater the loss. In the figure below, the area of the reverse recovery time triangle (hatched part) is the loss.
In the figures (a) and (b) below, (a) has a smaller triangle area, so it can be said that the loss is smaller.
The area of this triangle is determined by the switching time of the diode. In other words, the switching time also appears as a loss, affecting the efficiency of the power supply circuit.
Therefore, in general, a Schottky barrier diode with high-speed switching and a low VF value is used to reduce loss and improve power circuit efficiency.
"point"
- The diode acts as a low side switch.
- The diode forward voltage VF becomes a loss and affects efficiency.
- Losses also occur during diode switching.
- A high-speed, low-VF Schottky barrier diode is used.
2. Forward voltage: VF
When the high-side switch in the converter IC is OFF, current flows through the low-side diode. At this time, VF is generated because the diode is in the forward direction. Since VF is a loss, it deteriorates power supply efficiency. Therefore, select a low VF diode.
However, Schottky barrier diodes with low VF tend to have a large reverse current (IR). On the other hand, the larger the average rectified current (IO), the lower the VF. In that case, the package will be larger and the cost will increase at the same time, so we must not simply judge by VF alone, but also consider the balance with various parameters.
A guideline is 0.6V or less. (However, if there is a recommended value for the converter IC, it will take precedence.)
Since VF depends on the IF, it is necessary to check the VF value at the current value of the power supply specifications with a data sheet, etc. Specifically, use the VF-IF characteristics listed in the diode datasheet as shown in the figure below (from the RBR5LAM40A datasheet). In this figure, extract and check the VF value from the assumed current value (IF value).
* Check here for the latest data sheet of RBR5LAM40A. (Link to ROHM website)
Now let's use LTspice simulation to see how the VF characteristics affect the efficiency of the power supply circuit. A large VF should degrade efficiency.
Use the LTspice sample circuit model (LT3995_TA01A_DC1832A.asc) for the LT3995 on DC1832A and check the power efficiency in the "Efficiency Report" after simulation.
On the example circuit, the diode of interest is D1. The default is "B560C", but replace it with two diodes with different VFs to simulate.
Use RBR5LAM40A (VF=0.53Vmax) as a diode with an appropriate VF value, and RB088LAM150 (VF=0.9Vmax) as a diode with a high VF value.
Here, in order to clarify the difference in characteristics, we purposely choose a diode with a large VF value.
Derive the VF value from the forward current (IF) of each diode. Extract the value for IF=3A from the VF-IF characteristics listed in the datasheet of each diode.
* Check here for the latest data sheet of RBR5LAM40A. (Link to ROHM website)
* Check here for the latest datasheet of RB088LAM150. (Link to ROHM website)
The figure below shows the simulation results for each. For the RBR5LAM40A it is 85.8% efficient and for the RB088LAM150 it is 81.3% efficient. From this, it can be confirmed that increasing the VF value by approximately 0.32V reduces efficiency by approximately 4.5%.
(a) For RBR5LAM40A
(b) For RB088LAM150
"point"
- Since VF becomes a loss, select a low VF product. (Simulation results show that if the VF value is increased by approximately 32V, the efficiency drops by approximately 4.5%.)
- Low VF products tend to have a large IR.
- Those with large IO also tend to have low VF, but the larger the package, the higher the cost.
- As a guideline for the VF value, select 6V or less considering the balance between IR and IO (package size).
- Since the VF value depends on the forward current IF, extract the VF value that matches the current value from the VF-IF characteristics.
- If the VF value is high, the power (loss) consumed by the diode will also increase, causing heat generation.
3. Average rectified current: IO
For the IO value, select 1.2 times or more of the load current (forward current flowing through the diode: IF) as a guideline. However, if you use a large-capacity output capacitor for the converter, be aware that the current value will temporarily increase due to the rush current that occurs when the capacitor is charged when the power is turned on.
Now let's check the IO value using LTspice. The diode uses RBR5LAM40A. IO checks the current through L1 (inductor / coil) in the circuit. Load current checks the current flowing through Rload. In this result, the load current is about 3.0A, so 3.6A or less, which is 1.2 times that, can be said to be a reasonable value.
"point"
- Select an IO value that is at least twice the forward current IF as a guideline.
- If a large output capacitor is used, the inrush current at start-up should also be considered.
[Trivia]
If an inrush current occurs, check the characteristic value of the peak forward surge current (IFSM). The IFSM is listed in each product's datasheet. Refer to the IFSM-t characteristics (see the figure below) of the characteristic curves listed in the data sheet, and select according to the magnitude (peak current) and length (time) of the inrush current.
* Check here for the latest data sheet of RBR5LAM40A. (Link to ROHM website)
4. Reverse voltage: VR
VR is the reverse voltage of the diode. In this case, no current flows, but spike noise (overshoot) occurs when the forward and reverse directions are switched.
The figure below is a simulation result of LTspice that intentionally generates spike noise (overshoot) during rectification using a diode (RBR5LAM40A). Although it is not so extreme in reality, spike noise (overshoot) does occur to some extent.
Select a VR value that is at least twice the input voltage (maximum value) of the power supply circuit, taking into account spike noise (overshoot) in switching waveforms, etc.
"point"
- VR should be selected with a guideline of at least twice the input voltage (MAX value) of the converter.
5. Reverse current: IR
IR is the current that flows when a voltage is applied in the reverse direction. It is called reverse leakage current, or simply leakage current.
IR becomes a loss and affects efficiency. Therefore, the IR should be selected as small as possible. However, as a characteristic of Schottky barrier diodes, low VF devices tend to have large IR. Therefore, the order of priority for selection is to prioritize VF first, then select low VF products with small IR.
On the other hand, since IR fluctuates depending on the environmental temperature (especially at high temperatures), it is necessary to consider the environmental temperature for devices that are expected to operate in high temperature environments. The figure below shows the VR-IR characteristics extracted from the datasheets of RB058LAM100 and RBS3LAM40C, which have different temperature characteristics.
* Check here for the latest data sheet of RBS3LAM40C. (Link to ROHM website)
* Check here for the latest data sheet of RB058LAM100. (Link to ROHM website)
When selecting the actual circuit constants, the IR is extracted from the VR-IR characteristics listed in each product's data sheet and checked according to the assumed environmental temperature.
Using RBS3LAM 40C, let's look at changes in IR with LTspice at environmental temperatures of 25 °C and 100 °C. Green is 25 °C and blue is 100 °C. Comparing the IR (negative side area), it can be confirmed that 100 °C has increased.
Next, we will use LTspice simulation to check how the temperature characteristics of the IR change depending on the ambient temperature. IR should increase at higher temperatures. Again, we will use the LTspice sample circuit model of the LT3995 on the DC1832A, but we will compare the two types of diodes that have different IR temperature characteristics. One is RB058LAM100 and the other is RBS3LA M 40C with large IR. Also, change the input and output voltage of the power supply. Modify the circuit so that VIN is 10V and VOUT is 5V. Also try changing the load current I(Rload) to check the power efficiency. The temperatures are 25 °C and 100 °C.
(1) Iout=3A, ambient temperature 25°C in the case of
<< RB058LAM100》
* Check here for the latest data sheet of RB058LAM100. (Link to ROHM website)
<< RBS3LA M40C >>
* Check here for the latest data sheet of RBS3LAM40C. (Link to ROHM website)
(2) Iout=0.1A (light load), ambient temperature 100℃ in the case of
<< RB058LAM100》
* Check here for the latest data sheet of RB058LAM100. (Link to ROHM website)
<< RBS3LA M40C >>
* Check here for the latest data sheet of RBS3LAM40C. (Link to ROHM website)
In the case of the RBS3LA M40C, which has a large IR, it can be confirmed that the power efficiency drops dramatically from 91.1% to 58.2% when the ambient temperature reaches 100 °C.
Also, in the case of RB058LAM100, although it is not as extreme as RBS3LA M40C, we can confirm that it has decreased from 88.5% to 86.4%.
"point"
- IR (leakage current) becomes a loss.
- As a characteristic of Schottky barrier diodes, devices with low VF tend to have large IR. Therefore, VF should be prioritized, and then select a low VF product with a small IR.
- Since the IR fluctuates depending on the ambient temperature (especially at high temperatures), it is necessary to consider the ambient temperature for devices that are expected to operate in a high temperature environment.
How did you feel about "How to select a catch diode in an asynchronous rectification converter circuit?"
In the next issue, I plan to contribute about "Part 2: Selecting Input/Output Capacitors for Synchronous Rectification Converter Circuits."
Improve your circuit design skills by understanding the characteristics of discrete components!
In recent years, the design period has become tight due to the early introduction of products to the market. Even with proven designs and reference designs, discrete components must be selected for circuit optimization. At that time, this technical article will tell you the selection method that will be your base.