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H-Bridge-Slide7

All three of these devices are essentially pin for pin compatible. What is shown here is essentially the block diagram for the VNH2SP30 and the VNH3ASP30. There is the current sense feedback pin at the lower left of the device block diagram. Current sense is essentially a proportional current from the high side outputs. The current sense pin reflects a small fraction of the current that is coming from either high-side driver. An H-Bridge can be driven neatly with just a few pins from the micro. A limiting factor for the VNH2SP30 and the VNH3SP30 is that they are both only 16V capable. They have a 41V breakdown, but they only operate to 16V. The VNH3ASP30 is the analog feedback version of the VNH3SP30. It has an operating voltage of up to 36V which is well suited for industrial applications. To review, the 30A max rating is to deliver that amount of current but it cannot be driven to 30A. If it is driven that high it would overheat and melt off the board or shut down. Other protections consist of over voltage and thermal shutdown protection. There is thermal shutdown protection for both the high-side and low-side switches. There is also cross-conduction protection. The high-side and low-side cannot be turned on intentionally. The cross conduction protection circuit prevents that from happening. However, the part does have some issues with PWM-ing. PWM-ing is done with these devices by the use of the PWM pin. When the PWM pin is low, the low-side MOSFETs on both sides are forced off. If there is current flowing from upper right to lower left then the low side drivers are forced off, then the current through the lower left will flyback up through the upper left transistor body diode. When the PWM pin goes high again the lower left transistor turns back on. When the lower left transistor turns back on, the reverse recovery time of the upper left transistor body diode gets in the way. The upper transistor body diode reverse recovery time is dismal for this device. This can be compensated by using Schottky diodes across both of these high-side drivers. The Schottkey diodes save about half of the switching issues with the high side switches. The other half comes from the upper transistor having trouble staying off when the lower transistors turn back on. This has to do with the capacitance associated with the upper transistor gates. These capacitances are between the source and the gate and the drain and the gate. When the lower transistor turns on it pulls the source of the upper transistor fairly quickly, which it does because there is only one speed for this MOSFET. This causes the upper MOSFET to literally turn back on momentarily by “capacitor division”. This is called shoot through which translates to power dissipation. These devices can do some PWM-ing at higher voltages. Unfortunately, it is not going to work at 36V. There is too much shoot-through, even with the Schottkey diodes, and the part will overheat. So at lower voltages, 16V operation, at say 16kHz, designers might be able to get away with PWM-ing if the load is 5A or less. This issue is a limiting factor when considering PWM-ing loads. These devices are very good for DC H-Bridge applications, but, if users want to PWM with these parts, it is probably not advisable if he or she is looking for 20kHz and high current or higher voltages.

PTM Published on: 2014-12-19