Answer5

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	WHAT'S YOUR ENGINEERING QUOTIENT?
•What's the Count? Problem 5—The Zetex ZHB6718 is a full H-Bridge chip implemented by BJTs. The VCE(sat)s (saturation voltages across the collector and emitter) of the PNP and NPN BJTs are -0.2V and 0.15V respectively. The Fairchild NDS8858 is a half H-Bridge chip implemented by MOSFETs. The RDS(on)s (drain to source resistance at on-state) of the P-Channel and N-Channel MOSFETs are 0.10 and 0.055 Ohms respectively. Using a DC motor operating at 6V (assume constant voltage) and coil resistance of 12 Ohms, compute the following: ı Stall current of the DC motor switched by the Zetex chip (10 mgp) ı Stall current of the DC motor switched by the Fairchild chips (two half H-bridges make one full H-bridge) (10 mgp) ı Power dissipated at the Zetex chip when the motor is stalled (6 mgp) ı Power dissipated at the Fairchild chips when the motor is stalled (6 mgp) Answer: The stall current for the Zetex chip is (6V-0.2V-0.15V)/12Ohm=0.47A Remember that the 0.2V and 0.15V drop across the collectors and emitters of the BJTs! Similarly, the stall current for the Fairchild chips is 6V/(12Ohm+0.10Ohm+0.055Ohm)=0.49A This time, it's the on-resistance that you need to take into account. The power dissipated at the Zetex chip is 0.47A(0.2V+0.15V)=0.16W. The power dissipated at the Fairchild chips is 0.49A0.49A(0.10Ohm+0.055Ohm)=0.037W.* 12-99 For questions or comments about Test Your EQ, e-mail eq@circuitcellar.com.