As shown in the picture, the picture shows the internal block diagram of the commonly used BUCK chip.
There are three questions to ask you: 1. The bootstrap capacitor between BST and SW is generally 0.1uF. Is this value an empirical value or how to get it?
2. The way to charge the bootstrap capacitor is to charge it through a diode through VCC. Isn't the instantaneous current very large when the capacitor is charged? Will it burn out the diode? Do you need to connect the bootstrap capacitor in series with current limiting Resistance, how much resistance in series?
3. If the upper tube and the lower tube are externally installed, can the same model of Nmos tube be selected, that is, the parameters are the same. What should I pay attention to when choosing?
Savannah Posted on September 15, 2020
This is the bootstrap capacitor, used to power the driver,,,
Jeremiah Posted on September 15, 2020
The diode is inside the IC, and there is no resistance in series.
If it is connected to the outside, it is not connected to the diode.
Luis Posted on September 15, 2020
The string is connected to the bootstrap capacitor. I think there are strings in some places, and there are no strings. I personally understand that the series resistance can buffer the rising edge overshoot when the upper tube is turned on. I don’t know if it has any effect in this respect. .
Blake Posted on September 15, 2020
1. The value of the boot capacitor is generally 0.01-10uf, and most circuits generally recommend choosing 0.1uf or 1uf. The boot capacitor can be calculated. You need to know the gate charge, operating frequency, quiescent current and other parameters of the high-end power tube. This capacitor Generally, the recommended value is fine;
2. If it is an external bootstrap diode, the recovery time, current, etc. need to be considered when selecting the type. If it is built-in, as long as the boot capacitor selection is within the recommended range, there is no need to consider too much;
3. The external upper and lower tubes can be of the same model. Generally, the output current is greater than 10A, and external power tubes are mostly used. The maximum working current, withstand voltage, Rds value, and power consumption value should be considered when selecting the model. If the output current is large When selecting the type, the lower tube is generally larger than the upper tube, or the lower tube uses two parallel connections, because the switch damage of the upper tube accounts for the majority, the conduction loss of the lower tube accounts for the majority, and the loss of the lower tube is larger than that of the upper tube.
