AN2644 datasheet(49/64 Pages) STMICROELECTRONICS

49 / 64 page

AN2644

Power MOSFET driving energy in ZVS operation

49/64

The grey-shaded area below the gate charge curve represents the energy lost in the gate

driver to discharge the gate from VGS to zero. Its value can be simply found from the

difference of the total driving energy, represented by the rectangle enclosed by the Q-V axes

and the Qg and VGS lines:

Equation 20

and EON_HS:

Equation 21

In case of ZVS, instead, the associated gate-charge curve is the red line. Its slope is equal

to that of the third portion of the normal gate-charge characteristic, because CGD has the

final value since the beginning. By geometrical considerations, the total gate charge QgZ to

provide in this case will be:

Equation 22

while the energy lost in the gate driver to charge the gate up to the final value VGS and turn

the power MOSFET fully on and represented by the red-shaded area will be:

Equation 23

At turn-off, the Miller effect will be present but to a lower degree. In fact, the operating point

will initially move along the red line until the gate voltage reaches the Miller plateau VM (that

associated to the switched current at turn-off). From that point on, it will move along the

black curve. The energy associated to turn-off will be:

Equation 24

and the total driving energy:

Equation 25

For the power MOSFET used in the example of Figure 29 (STP14NK60Z), the datasheet

specifies: Qgs = 13.2 nC, Qgd = 38.6 nC, Qg = 75 nC @ VGS = 10 V. Substituting these

values and VM = 5.8 Vin (Equation 19) to (Equation 25) we find: EON_HS = 305 nJ, EHS = 750

nJ, EOFF_HS = 445 nJ, QgZ = 55 nC, EON_ZVS = 276 nJ, EOFF_ZVS = 331 nJ and EZVS = 607

nJ. Then, the total gate charge is decreased by 20 nC (-27%) and the total gate driving

energy by 143 nJ (-19%).

Qg V

⋅

OFFHS

Qg V

ONHS

–

⋅

–

------------------------- Qg

Qgs

Qgd

()

–

[]

ONZVS

---Qg

ZVGS

---

–

------------------------- Qg

Qgs

Qgd

()

–

[]

⋅

OFFZVS

–

()

---------------------------------- V

() Qg Qgd

–

() V

GS VGS

()Qgs

–

[]

ZVS

–

()

---------------------------------- 2V

() Qg Qgd

–

() V

GS 2VGS

()Qgs

–

[]

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