What's the most important factor that limits the maximum switching frequency for the UCC217XX gate driver family?
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What's the most important factor that limits the maximum switching frequency for the UCC217XX gate driver family?
TL;DR: The maximum switching frequency for the UCC217XX family is usually limited by thermal. To calculate the maximum switching frequency, use the UCC217XX XL calculator!
Introduction
The UCC21750 datasheet specified that the maximum switching frequency is theoretically 1MHz; however, it's hard to reach this maximum switching frequency in real-life applications because of the gate driver's thermal properties. To calculate the max switching frequency for a certain application, it's important to understand the average current and power dissipation for the gate driver.
Average current/power dissipation
The average current and power dissipation consist of two parts: the switching current and power dissipation, and the quiescent current and power dissipation.
When the gate driver is switching at high frequency, current from the secondary side power supply charges the gate capacitance of the transistor. The average current during this process can be calculated by:
where Qsw is the gate charge, Fsw is the switching frequency, and Ciss is the gate capacitance.
This gate charging current passes through the internal pull-up and pull-down resistance and causes power dissipation inside the gate driver circuit, causing the chip to heat up. The switching power dissipation inside the chip can be calculated with the UCC217XX calculator as well as the formula below in the datasheet:
Beside the switching power dissipation, the quiescent power dissipation also affects the temperature of the gate driver. The quiescent current on the secondary side is typically 4mA according to the datasheet, so the secondary side quiescent power dissipation is:
Thus, the total power consumption inside the gate driver can be calculated by
How these affect the maximum switching frequency
From the above total power consumption calculation, we can see that as the switching frequency increases, the total power consumption increases as well. The total power consumption, combined with the junction-to-board characterization parameter and the ambient temperature, gives the junction temperature of the board. The maximum junction temperature for UCC217XX is 150°C, and that is the factor that limits the switching frequency.
The formula to calculate junction temperature is shown below:
With being the junction-to-board characterization parameter in the datasheet, which is 32.3°C/W for UCC217XX. However, this value is calculated with the JESD51-6 standard under specific test condition; in actual application, the junction-to-board characterization parameter might vary, causing the junctional temperature to vary as well. Thus, it is recommended to leave plenty of headroom on thermal during design.
To learn more about the IC thermal metrics, check out this document.