Buck Boost Transformer Calculator

Calculate key parameters for a buck-boost transformer configuration.

Understanding Buck-Boost Transformers

A buck-boost transformer is a type of DC-DC converter that can produce an output voltage that is either higher or lower than the input voltage. Unlike simple buck (step-down) or boost (step-up) converters, the buck-boost topology can achieve both functions, often with a polarity inversion (output voltage is negative relative to the input ground, though non-inverting buck-boost converters also exist). This calculator focuses on the fundamental non-inverting buck-boost configuration which requires a transformer for voltage step-up or step-down and can achieve a regulated output.

Key Parameters and Formulas:

The operation of a buck-boost converter relies on controlling the switching of an inductor (or a transformer's primary winding acting as an inductor) to store and release energy. The key parameters calculated by this tool are:

• Duty Cycle (D): The fraction of the switching period for which the switch is ON. For a non-inverting buck-boost converter (assuming ideal conditions with transformer turns ratio n=1, and neglecting losses):
  D = |Vout / (Vin + Vout)|
  If an input duty cycle is provided, the calculator will use it.
• Inductance (L): The inductance required for continuous conduction mode (CCM), which ensures smooth current flow and efficient operation. A common formula for inductance, considering a desired ripple current (ΔIL), is:
  L = (Vin * D) / (Fs * ΔIL)
  Here, we will calculate the minimum inductance required for CCM, by assuming a ripple current of 20% of the peak inductor current (IL_peak).
  IL_peak = Io / (1 - D) (for a basic non-inverting buck-boost)
  ΔIL = 0.20 * IL_peak
L_min = (Vin * D) / (Fs * ΔIL) = (Vin * D) / (Fs * 0.20 * (Io / (1 - D)))
L_min = (Vin * D * (1 - D)) / (Fs * 0.20 * Io)
  Note: Transformer ratio 'n' affects Vin and Vout relations. For a simple buck-boost analysis, we assume Vin and Vout are terminal voltages. For transformer-based buck-boost, the primary side voltage is Vin, and secondary side voltage relates to Vout via the transformer ratio. This calculator assumes Vin and Vout are the desired input and output voltages *after* any ideal transformer action. For simplicity in this calculator, we assume an ideal transformer with a turns ratio that facilitates the desired Vout from Vin, and calculate parameters based on the effective Vin and Vout. A common approximation for a non-inverting buck-boost with transformer is that the required duty cycle (D) and inductance calculation relies on the total voltage swing.
  For an non-inverting buck-boost with transformer, the relationship between input and output voltage is often expressed as: Vout = -n * Vin * (D / (1 - D)) or similarly depending on winding configuration. If we consider Vin and Vout as magnitudes and allow for non-inversion, a simplified approach for D is: D = Vout / (Vin + Vout) (assuming ideal transformer and ideal switches)
  And the inductance formula based on this D and peak current:
  IL_peak = (Vin * D) / (Fs * L) (This relates peak current to Vin and L for the ON time)
  Or, considering the output current:
  Io = IL_peak * (1 - D) (This assumes the output current is drawn only during the OFF time, simplified)
  Rearranging for L: L = (Vin * D) / (Fs * (IL_peak - Io))
  Using IL_peak = Io / (1-D), we get L = (Vin * D) / (Fs * (Io / (1-D) - Io))
L = (Vin * D * (1 - D)) / (Fs * Io). This is a common simplification for buck-boost inductance calculation.
• Peak Switch Current (Is_peak): The maximum current that the switching element (e.g., MOSFET) must handle.
  Is_peak = Io / (1 - D)
• Peak Diode Current (Id_peak): The maximum current that the freewheeling diode must handle. In many basic buck-boost analyses, this is similar to the peak switch current.
  Id_peak = IL_peak = Io / (1 - D) (Simplified, assuming diode carries full inductor current when switch is off)
• Transformer Turns Ratio (n): If the desired output voltage cannot be achieved with D=0.5 and Vin, a transformer is used. For a non-inverting buck-boost, the relationship is typically:
  Vout = -n * Vin * (D / (1 - D)) (for inverting)
  For non-inverting configurations, the transformer winding and connection are crucial. If we treat this as a generalized converter where Vin and Vout are positive, and D is calculated as D = Vout / (Vin + Vout), then the effective AC voltage the inductor/primary sees relates to Vin. If Vin != Vout, a transformer might be assumed implicitly to enable this ratio or to manage current. However, without explicit transformer configuration details, we calculate L and currents based on effective Vin, Vout, and D. If Vin and Vout are very different and D is close to 0 or 1, the required inductance can become very large or very small, and the peak currents can be extreme.

How to Use This Calculator:

1. Enter the Input Voltage (Vin) in Volts.
2. Enter the Desired Output Voltage (Vout) in Volts.
3. Enter the Load Current (Io) in Amperes.
4. Enter the desired Switching Frequency (Fs) in Kilohertz (kHz).
5. Optionally, enter the Duty Cycle (D) in percent (%). If left blank, the calculator will compute it based on Vin and Vout using the formula D = Vout / (Vin + Vout). This assumes ideal conditions and a non-inverting buck-boost topology where Vin and Vout are positive magnitudes.
6. Click "Calculate Parameters".

Example Calculation:

Let's calculate the parameters for a common scenario:

• Input Voltage (Vin): 12V
• Desired Output Voltage (Vout): 5V
• Load Current (Io): 2A
• Switching Frequency (Fs): 100kHz

If we don't input a Duty Cycle:

• Calculated Duty Cycle (D) = 5 / (12 + 5) = 5 / 17 ≈ 0.294 or 29.4%
• Peak Switch Current (Is_peak) = 2A / (1 - 0.294) ≈ 2A / 0.706 ≈ 2.83A
• Required Inductance (L) (assuming 20% ripple) = (12V * 0.294 * (1 - 0.294)) / (100,000 Hz * 0.20 * 2A) = (12 * 0.294 * 0.706) / (40,000) ≈ 2.49 / 40,000 ≈ 0.000062 H = 62 µH

The calculator will output these derived values.

Important Considerations:

This calculator provides theoretical values based on simplified formulas. Real-world implementations will differ due to:

• Component Losses: Resistors, switches (conduction and switching losses), diodes (forward voltage drop), and inductor (core and copper losses) all reduce efficiency.
• Non-Ideal Components: Inductor non-linearity, capacitor ESR (Equivalent Series Resistance), parasitic inductances and capacitances.
• Transformer Modeling: Accurate transformer modeling involves magnetizing inductance, leakage inductance, and winding resistances, which are not included in this basic calculation. The transformer's turns ratio (n) plays a critical role in achieving the desired Vout from Vin. This calculator simplifies by assuming an effective Vin and Vout and calculating based on a derived duty cycle.
• Control Loop Stability: Feedback control systems are necessary for stable regulation under varying loads and input voltages.
• Ripple Current Selection: The choice of ripple current (ΔIL) affects the required inductance and peak currents. Lower ripple requires higher inductance but reduces peak currents and potentially core losses.

Always use calculated values as a starting point and consult datasheets for chosen components. Consider safety margins for voltage and current ratings.

Calculated Parameters

Duty Cycle (D):

Peak Switch Current (Is_peak):

Peak Diode Current (Id_peak):

Minimum Inductance (L_min @ 20% ripple):