Technology / Technical guide
Gate Charge and Driver Power: Define the Supply Model
A voltage-controlled power device can need repeated charge transfer at its control electrode. Gate-drive supply power concerns that charge-transfer path. It is distinct from the main power path’s conduction and switching losses, and cannot be reduced to one universal gate-capacitance number.
Charge, drive swing and repetition rate belong to one declared model.
Identify what energy enters the drive supply before deciding where it is stored or dissipated.
01 / Charge quantity
Qg refers to a defined transition and operating condition.
Gate charge is an amount of charge associated with changing the gate state under specified device and circuit conditions.
Gate charge is an amount of charge associated with changing the gate state under specified device and circuit conditions. It has units of coulombs. It is not a capacitance in farads, although a constant linear capacitor would satisfy Q = CV for a defined voltage change.
A real semiconductor’s gate-related capacitances can depend on voltage and interact with the drain transition. Therefore one charge value cannot be treated as a universal constant for every voltage swing or power-path condition. The Miller-coupling guide develops that interaction separately.
02 / Supply model
Declare a nonrecovering unipolar charge cycle.
Consider a simplified fixed positive drive supply Vdrive, a gate charged from its reference to that supply each cycle, and discharge back to the reference without energy recovery.
Consider a simplified fixed positive drive supply Vdrive, a gate charged from its reference to that supply each cycle, and discharge back to the reference without energy recovery. Assume charge Qg is drawn from this supply during each full charge transition, under unchanged operating conditions.
Electrical energy drawn for that charge transition is Ecycle = VdriveQg. With repetition frequency frep, dynamic supply power is Pdynamic ≈ VdriveQgfrep. TI’s reference-design calculation uses this charge-voltage-frequency relationship and separately includes other supply consumption. The expression is not a complete universal gate-driver model.
03 / Arithmetic example
Keep charge units and cycle counting explicit.
Hypothetically let Qg = 20 nC, Vdrive = 5 V and frep = 1,000 cycles/s in the stated supply model.
Hypothetically let Qg = 20 nC, Vdrive = 5 V and frep = 1,000 cycles/s in the stated supply model. Energy drawn per cycle is 100 nJ, and dynamic average power is 0.0001 W, or 0.1 mW. These invented quantities illustrate arithmetic only.
The full charge-and-discharge cycle is counted once in that expression, rather than automatically multiplying by two for its two edges. Counting transitions under another supply arrangement may differ. None of these assumed values is a MARSTON drive voltage, switching rate, charge rating or controller setting.
04 / Energy destination
Supply energy does not all become driver-chip heat.
For a hypothetical constant ideal gate capacitance C charged from zero through a dissipative path by a fixed V supply, the supply provides CV².
For a hypothetical constant ideal gate capacitance C charged from zero through a dissipative path by a fixed V supply, the supply provides CV². Half becomes stored field energy and half is dissipated during charging. Subsequent nonrecovering discharge dissipates the stored half.
This familiar capacitor example explains why supply energy over a full cycle differs from the maximum stored energy. A real gate network can distribute heat among driver output stages, external resistance and device-related paths. Dynamic supply power is therefore not automatically the driver IC’s own dissipation.
05 / Other contributions
The main switch’s losses have another boundary.
Quiescent consumption, leakage, isolation circuitry and supply-conversion losses can add to the drive account.
Quiescent consumption, leakage, isolation circuitry and supply-conversion losses can add to the drive account. Bipolar swings, partial transitions or energy-recovery drives need their own charge and supply-energy relationships. A single VdriveQg term does not establish those contributions or arrangements.
Main-device switching loss instead concerns energy associated with the main voltage and current during transitions. The switching-loss guide explains that boundary. Gate-driver demand and main-path loss can interact, but one cannot simply substitute for the other.
06 / Product scope
A charge model does not reveal a converter implementation.
Useful interpretation retains the charge-curve conditions, supply reference, counted cycle, repetition rate and included consumption paths.
Useful interpretation retains the charge-curve conditions, supply reference, counted cycle, repetition rate and included consumption paths. Comparing charge numbers from unlike conditions can conceal differences in the actual transitions. The simplified model helps explain proportionality rather than nominate a device or operating rate.
No power-device type, gate-driver circuit, supply voltage or Qg value is confirmed for MARSTON equipment here. This guide provides no gate resistance, timing adjustment or connection instructions. It explains energy accounting without asserting implemented switching technology or an efficiency result.
Related technical guides: Snubber Concepts: Transient Energy, Peak Voltage and Damping · Dielectric Loss Tangent: Phase, Frequency and Equivalent Models · Signal Debouncing: Persistence, Sampling & Event Timing.
General background: Texas Instruments: Gate-Driver Power Requirement. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Gate charge | Qg in C | Specified transition and operating conditions. |
| Drive supply | Vdrive in V | Fixed positive nonrecovering unipolar model. |
| Energy per cycle | VdriveQg in J | Charge drawn once from the declared supply. |
| Dynamic supply power | VdriveQgfrep in W | Repeated identical complete cycles. |
| Heat allocation | Driver versus other paths | Supply power is not solely driver-chip dissipation. |
Engineering questions.
Is gate charge a capacitance?
No. Charge is in coulombs; capacitance relates charge change to voltage under a specified model.
Should the displayed power formula always be multiplied by two?
No. It already counts one full repeated charge-and-discharge cycle under the declared model.
Is all drive power dissipated inside the driver IC?
No. Energy can be dissipated across several paths.
Does the formula include quiescent supply power?
No. Additional consumption terms require their own account.
Can the same formula be assumed for an energy-recovery drive?
No. Its supply-energy exchange needs the applicable model.
Are 5 V and 1,000 cycles/s product instructions?
No. They are hypothetical values for checking units and arithmetic.
Technical enquiry
Discuss the technical question.
A voltage-controlled power device can need repeated charge transfer at its control electrode. Share the model and applicable operating or design information for an engineering discussion.