Another method for driving MOSFET-bridges is the use of a specialised transformer known as a GDT (gate drive transformer), which gives the isolated outputs for driving the upper FETs gates. The transformer core is usually a ferrite toroid, with 1:1 or 4:9 winding ratio. However, this method can only be used with high frequency signals. The design of the transformer is also very important, as the leakage inductance should be minimized, or cross conduction may occur. The outputs of the transformer are usually clamped by Zener diodes, because high voltage spikes could destroy the MOSFET gates.