If you are planning to make your own inverting buck-boost but don’t know where to start, therefore this inverting buck-boost step by step design guide is right for you. A buck-boost converter is a DCDC switching converter that combines the function of a buck and boost converter. Inverting buck-boost is a variant wherein the output is negative with respect to the ground.

**Detailed Inverting Buck-Boost Step by Step Design Guide**

**1. Supply the Known Parameters**

Start by defining the basics like input and output values. They must be given during the design stage.

**Example**:

Vin = 12V, Vout = -5V, Iout = 11A

**Where;**

- Vin – is the input voltage of the buck-boost converter
- Vout – is the output voltage
- Iout – is the load current

**2. Compute the Ideal Duty Cycle**

Buck-boost is a duty cycle controlled DCDC converter. Once you able to derive the duty cycle, you can compute the rests of the important parameters.

**Duty = – Vout / (Vin – Vout )**

Using above given;

Duty = – Vout / (Vin – Vout ) = – (-5V) / [12V – (-5V)] = **29.412%**

**3. Define the Switching Frequency**

You need to define what switching frequency level you will set the converter. The defining factor in selecting the switching frequency is power density, controller capability and EMI noise.

**Example:** Fsw = **250 kHz**

**4. Determine the Inductance Value**

**L1 = Ton X ( Vin – VQ1 ) / ( %di X Iout )**

- Ton = Duty / Fsw = 0.29412 / 250kHz =
**1.18 usec** - VQ1 – this is the voltage drop of the switch. Example: VQ1 =
**0.2V** - %di –This is the set level of the inductor ripple current. This must be provided during design stage. A good rule of thumb is 20%-40% of the load current. Example: %di =
**25%** - Iout – this is the output current declared above

Thus;

L1 = Ton X ( Vin – VQ1 ) / ( %di X Iout ) = 1.18 usec X ( 12V – 0.2V ) / ( 0.25 X 11A ) = **5.06 uH**

Consider a standard Inductor, say

L1_selected = **5uH**

**5. Select the Inductor**

**Inductor Peak Current**

**Inductor DC Current**

Idc_L1 = Imax – di_actual + di_actual / 2 = 16.972A – 2.78A + 2.78A / 2 = **15.582A**

**Inductor RMS Current**

Irms_L1 = [ di_actual / sqrt ( 3 ) ] + Imax – di_actual = [ 2.78A / sqrt ( 3 ) ] + 16.972A – 2.78A = **15.78A**

The selected inductor must have current rating that is higher to all the computed values above.

**6. Select a MOSFET Switch**

**Peak Current**

Imax = same inductor Imax above

**DC Current**

**RMS Current**

The selected switch must have a current rating higher than all the computed values above.

**Voltage Stress**

**VQ1_max = Vin + VD1 – Vout**

VD1 – this is the voltage drop of the diode. Example: VD1 = **0.7V**

Thus,

VQ1_max = 12V + 0.7V – ( -5V ) = **17.7V**

The selected MOSFET must have a voltage rating higher than this value with ample of margin.

**Power Dissipation**

**Pdiss_Q1 = Ploss_conduction + Ploss_switching**

**Ploss_conduction = Irms_Q1 X Irms-Q1 X RDSon_Q1**- RDSon_Q1 = on state resistance, Example: RDSon_Q1 =
**0.01 ohm** **Thus,**- Ploss_conduction = Irms_Q1 X Irms-Q1 X RDSon_Q1 = 8.568A X 8.568A X 0.01ohm =
**0.734W**

**Ploss_switching = Ploss_gatecharge + Ploss_COSS + Ploss_risefall**- Plos_gatecharge = ½ X Qgtotal X Vdrive X Fsw
- Qgtotal – this is the total gate charge indicated in the MOSFET datasheet. Example: Qgtotal =
**1nC** - Vdrive – this is the voltage applied to the gate to source of the MOSFET. Example: Vdrive =
**12V** **Thus,**- Ploss_gatecharge = ½ X Qgtotal X Vdrive X Fsw = 0.5 X 1nC X 12V X 250kHz =
**0.0015W** - Ploss_COSS = ½ X COSS X ( Vdrain_max )
^{2}X Fsw - COSS – this is the output capacitance of the MOSFET. Example: COSS =
**1nF** - Vdrain_max – this is the peak drain voltage. This is equal to the VQ1_max above.
**Thus,**- Ploss_COSS = ½ X COSS X ( Vdrain_max )
^{2}X Fsw = 0.5 X 1nF X ( 17.7V )^{2 }X 250kHz =**0.039W** - Ploss_risefall = 0.5 X ( trise + tfall) X Irms_Q1 X Vdrive X Fsw
- trise – this is the rise time of the MOSFET. See datasheet. Example: trise =
**1nsec** - tfall – this is the fall time of the MOSFET. See datasheet. Example: tfall =
**1nsec** **Thus,**- Ploss_risefall = 0.5 X ( trise + tfall) X Irms_Q1 X Vdrive X Fsw = 0.5 X ( 1nsec + 1nsec ) X 8.568A X 12V X 250kHz =
**0.025W**

**Finally,**

Pdiss_Q1 = Ploss_conduction + Ploss_switching = 0.734W + 0.0015W + 0.039W + 0.025W = **0.8W**

The selected MOSFET must have a power dissipation rating higher than this value with ample of margin.

**7. Select the Diode**

**Peak Current**

The diode peak current is the same to the peak inductor and MOSFET current.

Imax = same inductor Imax above

**DC Current**

The DC current of the diode is just equal to the output current

Idc_diode = Iout

**RMS Current**

The selected diode must able to handle all the computed currents above with ample of margin.

**Peak Reverse Voltage**

**PRV_D1 = Vin – VQ1 – Vout**

Using all the values declared above,

PRV_D1 = Vin – VQ1 – Vout = 12V – 0.2V – ( -5V ) = **16.8V**

The selected diode voltage must be higher than this with more margin.

**Power Dissipation**

**Ploss_diode = VF_diode X Irms_diode**

VF_diode – this is the forward voltage of the diode. See the datasheet. Example: VF_diode = **0.7V**

**Thus,**

Ploss_diode = VF_diode X Irms_diode = 0.7V X 13.273A =** 9.29W**

The selected diode must have a power rating higher than this with a good margin.

**8. Select the Output Capacitor**

**Ripple Current**

The selected capacitor must have a ripple current higher than this with ample of margin.

**Voltage**

The selected output capacitor must have a voltage rating higher than the output voltage by a enough margin.

**Minimum Capacitance**

Where;