Often use DC-DC we will find that DC-DC common switching frequency selection will be 500KHZ, 800KHZ, 2MHZ, 2.2MHZ, different switching frequencies correspond to different use scenarios, this section briefly describes the DC-DC switching frequency in principle and how it affects DC-DC performance.
1. Definition of switching frequency
Figure 21-1: Buck topology
As shown in Figure 21-2, for step-down DC-DC, the upper tube conducts the lower tube off, the inductor energy storage, the downtube conducts the upper tube off, and the inductor releases energy. The sum of the energy storage and release time of the inductor is a cycle TS, take its reciprocal, that is, the switching frequency of the switch tube f=1/TS, that is, the switching frequency measures the length of the sum of the opening time + off time of the switching element, the lower the frequency, the longer the time, the larger the frequency, the shorter the time.
Figure 21-2: Switching frequency definition
2. Switching frequency and inductive capacitance
NEXT, WE SELECTED 500KHZ, 1MHZ AND 2MHZ FOR ANALYSIS. As shown in Figure 21-3, the same step-down DC-DC, Vin=12V, Vout=6V, are set to three operating frequencies, namely f1=2MHZ, f2=1MHZ, and f3=500KHZ.
Figure 21-3: Comparison of operating frequencies
Because Vout is all 6V, according to the duty cycle buck principle, SW1, SW2, and SW3 have a duty cycle of 0.5. From this point, it can be seen that the difference in operating frequency has nothing to do with the size of the output voltage.
The higher the operating frequency, the higher the inductor ripple current and inductor ripple voltage frequency, but the amplitude will decrease, and the capacitance and ESR requirements for the input and output capacitors will decrease.
The lower the operating frequency, the lower the inductor ripple current and inductor ripple voltage frequency, but the amplitude will rise, and the capacitance and ESR requirements for the input and output capacitors will increase, and the subsequent voltage/current waveforms will not be supplemented here. (Detailed waveform review portal: DC-DC-2: How the step-down type works)
SW on, inductor storage, SW off, inductor discharge, we define the inductor's storage and discharge as one working step of the inductor. The inductance value of the inductor determines the upper limit of the working step of the inductor, the time from 0 to full inductance is defined as TX, and the time from full value to release to 0 is defined as TF, then the limit working step of the inductor is TX+TF (in fact, it will definitely not be close to the limit working step, that is, it will not be full during energy storage, and it will not be empty during energy release). The smaller the inductance, the shorter its operating step, and the larger the inductor's inductance, the longer its operating step can be.
According to the above, the smaller the operating frequency of DC-DC, the longer the opening time + shutdown time of the switching element, the longer the working step of the inductor, and the corresponding inductance value must be increased, otherwise inductor saturation will occur.
The larger the working frequency of DC-DC, the shorter the opening time + shutdown time of the switching element, then the working step of the inductor does not need to be so large, and the corresponding inductance can be reduced, reducing the cost and volume of the inductor.
According to the most basic formula:
Although the ripple current amplitude can be reduced by increasing the inductor's inductance, the larger the inductor L's inductance, the longer its working step, and the slower the response speed, so that the dynamic response speed of the entire DC-DC loop becomes slower, so the value of the inductor cannot be too small or too large, and needs to be reasonably calculated: (Portal: DC-DC-15: The article teaches you how to calculate the inductance value of DC-DC).
Brief summary:
1: Under the same working conditions and current, voltage, ripple parameters, the frequency from small to large required component size and device
值为Cin(500KHZ)>Cin(1MHZ)>Cin(2MHZ),Cout(500KHZ)>Cost(1MHZ)>Cout(2MHZ),L(500KHZ)>L(1MHZ)>L(2MHZ)。
2: Switching loss and chip temperature rise. Although an increase in switching frequency can reduce the size of peripheral components, it also results in greater switching losses. Still look at Figure 21-3, when f=500Khz, a cycle SW switch 2 times, in the same time period, f=1Mhz, SW switch 4 times, f=2Mhz, SW switch 8 times. The more switching times, the greater the MOS heat loss, the lower the overall efficiency of DC-DC, and the temperature rise will also rise, at this time, if the external ambient high temperature is superimposed, DC-DC is easy to trigger overtemperature protection.
3. Switching frequency and minimum on-time
The minimum on-and minimum off-times of the DC-DC refer to the turn-on limits of the internal switching elements that are reached at the limit duty cycle.
Passenger cars with 12V battery systems are typically 9V-16V, while commercial vehicles with 24V battery systems have a wider range to 18V-32V. In fact, the minimum on-time and minimum off-time are generally not touched, because the commonly used step-down system is 12V--->5V, 24V--->5V, and only when the step-down ratio is closer to 1 or closer to 0, the minimum on-time and minimum off-time will be touched. Here are just two examples of limits:
Minimum on-time of the upper tube
Figure 21-4: Cases where the duty cycle is relatively small
For the minimum on-time of the top tube, it is more important to consider the presence of some large input-output voltage differences and high switching frequencies in the application. For example, in a commercial vehicle 24V battery system, the steady-state value of the input voltage will be as high as 32V, and the switching frequency will be as high as 2. 2MHZ, assuming that the system requires 3.3V power supply, that is, there is 32V to 3.3V, and the minimum on-time at this time:
The on-time of the upper tube is about 51ns, and it is necessary to pay attention to whether the minimum on-time of the selected DC-DC upper tube meets this parameter.
4. Switching frequency and minimum off-time
The minimum on-and minimum off-times of the DC-DC refer to the turn-on limits of the internal switching elements that are reached at the limit duty cycle.
Minimum shut-off time for upper tube
Figure 21-5: Cases with a large duty cycle
For the minimum turn-off time of the top tube, take into account the close operation of the input and output. For example, in the surround view controller, some electronic control units will give the voltage to the camera module will be 8V, and the input voltage range of the 12V electronic system is 9V to 16V, so when the input is 9V, the switching frequency is still assumed to be 2MHZ, and the minimum off-time at this time:
It is concluded that the shut-off time of the upper tube is about 55. 6ns。
Figure 21-6: Min on/Min off parameters in the data sheet
According to the data information of the chip shown in Figure 21-6, we can get that the upper limit of the minimum shutdown time of the chip is 60ns, so there is a hidden danger of touching the minimum shutdown time. If less than the minimum off-time, the chip adaptively reduces the switching frequency to meet the minimum off-time requirement. Therefore, if you want to keep the switching frequency stable and ensure that the electrical characteristics and EMI characteristics do not change, you can reverse the maximum operating frequency according to the duty cycle formula. Setting the switching frequency low to avoid triggering to the minimum off-time can be set to 1. 6MHZ or less.