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.
Read this article to review about buck converter.
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.
Vin = 12V, Vout = -5V, Iout = 11A
- 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
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. Inverting Buck-Boost Step by Step Design Guide – Select a MOSFET Switch
Imax = same inductor Imax above
The selected switch must have a current rating higher than all the computed values above.
VQ1_max = Vin + VD1 – Vout
VD1 – this is the voltage drop of the diode. Example: VD1 = 0.7V
VQ1_max = 12V + 0.7V – ( -5V ) = 17.7V
The selected MOSFET must have a voltage rating higher than this value with ample of margin.
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
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
- 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.
- 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
- 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
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
The diode peak current is the same to the peak inductor and MOSFET current.
Imax = same inductor Imax above
The DC current of the diode is just equal to the output current
Idc_diode = Iout
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.
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
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
The selected capacitor must have a ripple current higher than this with ample of margin.
The selected output capacitor must have a voltage rating higher than the output voltage by a enough margin.
Summarizing Inverting Buck-Boost Step by Step Design Guide
Above equations are being derived through Mathcad. Thus, the equations are exact. However, actual values may slightly differ to above equation results. This is acceptable because there are factors that present in the actual but not considered in the above analysis or equation derivations. In designing, what is more important is to select the right size components. So, always considers worst case scenario.