Power Tip New Short-Circuit Technology for Greater Pedelec Safety

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Pedelecs have become an environmentally friendly means of transportation. An electric motor supports pedaling and makes it easier to ride uphill. On downhill stretches, it can also be used to brake or feed energy back into the battery, provided it is mounted in the wheel hub. But watch out!

When the electric bike is rolling, the voltage generated by the motor must be prevented from damaging the electronics at all costs. This is where "active short-circuit technology" comes into play, which can also be used to safely dissipate large amounts of energy. It implements a braking function in which all high- and low-side MOSFETs are switched on. Figure 1 shows an e-bike architecture based on the DRV8363-Q1 gate driver.

In older systems, discrete components were used to detect when the permissible battery voltage was exceeded and to react accordingly. However, a dynamic reaction to any MOSFET failures was not possible here. The DRV8363-Q1 reduces the required PCB area and can activate an emergency braking mode in the event of failures.

It also triggers an active short-circuit circuit—either in response to an SPI command or automatically as soon as an overvoltage is detected. A low- or high-side braking mode can also be activated by setting the corresponding registers.

Problems With Braking

Several problems are conceivable during braking. Firstly, low-side braking can lead to a failure in a high-side MOSFET (or vice versa). In this case, a current path would be created from the 48 V supply to ground, which could damage the system and even pose a danger to users.

In such a case, the DRV8363-Q1 contains a function that triggers a switchover to the other braking mode so that the energy is dissipated in an orderly manner and a short circuit to ground is prevented. The result is an increase in safety, while the demands on the firmware are reduced.

High Current Peaks When Changing from Braking to Freewheeling

Another potential problem is high current peaks in the motor phase windings when switching from braking to freewheeling mode. This is where the improved responsiveness of the DRV8363-Q1 helps to regulate high voltages. In braking mode, there is also a risk of MOSFETs being damaged by overheating. Thanks to its ability to activate the high- or low-side braking mode via SPI, the DRV8363-Q1 can continuously switch between both operating modes to better distribute heat generation and thus facilitate thermal management of the PCB. Figure 2 illustrates the current flow in high-side and low-side braking mode.

Inaccuracies when measuring the voltages on the battery and on the MOSFET drains, as well as delays in capturing and decoding this information by the microcontroller, which has to activate the brake command, can be dangerous in emergencies. The DRV8363-Q1 therefore measures the battery directly at the drain connection of the high-side MOSFET, which improves the measurement accuracy and response time for triggering the braking mode in the event of overvoltages.

Braking Even With Hardware Faults

Finally, it can be problematic if the active short-circuit circuit cannot be activated due to an MCU defect or an error in the driver. The integrated active short-circuit function of the DRV8363-Q1 offers a more reliable braking method that is also available in the event of hardware faults. Even if a software or hardware error occurs in the MCU, the driver can automatically activate the active short-circuit circuit in the event of an overvoltage without an external command from the microcontroller.

Another not insignificant advantage of the chip is the fact that it replaces the discrete solution consisting of several components with an integrated circuit, which is important for pedelecs with their limited space and weight budget. (kr)

Akshay Rajeev Menon is an application engineer at Texas Instruments in Dallas, USA.

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