How Transformer Turns Ratio Really Affects Efficiency

Short version: in an ideal transformer, the turns ratio only sets voltage and current. In real ferrite-core transformers (SMPS, RF baluns, EFHW/OCF UNUNs…), the chosen ratio indirectly changes flux density, copper losses, and parasitics. So yes, turns ratio absolutely can influence efficiency — just not in the simplistic “more ratio = more loss” way you often see online.

This article focuses on ferrite-core transformers. The same ideas carry over to HF UNUNs and RF transformers, just with different frequencies and waveforms.

Turns ratio in plain language

What is “turns ratio”?

A transformer has two (or more) windings on the same core:

• Primary: connected to the input (source side)
• Secondary: connected to the output (load side)

Each winding has a certain number of turns:

• N_p = primary turns
• N_s = secondary turns

The turns ratio is simply “how many turns on the secondary compared to the primary?”

• Step-down example: 100:10 (primary:secondary)
• Mathematically: n = N_s / N_p

This ratio sets the voltage conversion:

• If n = 0.1 → secondary voltage is about 0.1 × primary voltage (step-down)
• If n = 5 → secondary voltage is about 5 × primary voltage (step-up)

Because power is (roughly) conserved, when you lower voltage, current goes up, and vice versa.

Ideal vs. real transformers

In a perfect transformer:

• No losses at all
• Efficiency = 100%
• Turns ratio changes voltage & current, but not efficiency

Real ferrite transformers (SMPS, RF baluns, EFHW/OCF UNUNs) are different:

• The core gets hot → core loss
• The copper gets hot → copper loss
• There is leakage inductance, stray capacitance, skin effect, proximity effect…

All of these are affected by:

• How many turns you use on each side
• How thick the wire or copper foil is
• How the windings are arranged
• And therefore, indirectly, the chosen turns ratio

How turns ratio affects efficiency in plain language

Think of three main “buckets” of loss:

Core loss

• Fewer primary turns → stronger magnetic swing in the core for the same voltage
• Stronger swing → more core loss and risk of saturation
• More primary turns → weaker swing, so core loss may drop
• But more turns means longer wire → more copper loss

Copper loss (I²R)

• Large step-down ratio (e.g. 50:1) means very high current on the low-voltage side
• That side then needs very thick copper, which eats window space and still has AC losses (skin & proximity effect)
• Too high current = more heating = worse efficiency

Layout & parasitics (especially in ferrite HF transformers)

• Very unequal turn counts (e.g. 100 turns vs. 1–2 turns) force awkward geometries
• This increases leakage inductance and stray capacitances
• Those parasitics create extra switching and ringing losses in the power electronics

Key idea: the turns ratio doesn’t change efficiency by itself, but it forces certain numbers of turns, current levels, and winding layouts — and those directly affect efficiency.

Practical examples

Example A – Gentle step-down (10:7)

• Currents are reasonable on both sides
• Both windings can use realistic wire sizes
• Winding is compact and easy to interleave
• Good chance of high efficiency

Example B – Extreme step-down (100:2)

• Primary needs many turns of thin wire → higher copper resistance
• Secondary has very few turns with huge current → thick conductors, serious AC losses
• Layout becomes messy, leakage and parasitics go up
• Efficiency can be noticeably worse, even if the “ideal transformer” math looks fine

In SMPS design you choose the ratio not only to hit the target voltage, but also to keep:

• Core flux density under control
• Currents within reasonable limits
• Winding layout practical and compact

That’s where efficiency is really won or lost.

Technical deep dive

Basic transformer relationships

Let:

• N_p = primary turns
• N_s = secondary turns
• n = N_s / N_p = turns ratio (secondary/primary)
• V_p, I_p = primary voltage and current
• V_s, I_s = secondary voltage and current

In an ideal transformer:

• V_s / V_p = N_s / N_p = n
• I_p / I_s = N_s / N_p = n → I_s = I_p / n
• P_in = P_out = V_p I_p = V_s I_s

For a real transformer, the efficiency is:

η = P_out / P_in = P_out / (P_out + P_loss)

where the total loss is:

P_loss = P_Cu,p + P_Cu,s + P_core + P_misc

Our goal is to see how the turns ratio n influences those loss terms.

Flux density and minimum primary turns

Ferrite cores in SMPS are usually driven with a square-ish waveform at high frequency (for example 20–200 kHz).

For a sinusoidal voltage, the classic relationship is:

V_p,rms = 4.44 · f · N_p · A_e · B_max

For a symmetrical square wave (typical ferrite SMPS):

V_p,rms ≈ 4 · f · N_p · A_e · B_max

where:

• f = switching frequency
• A_e = effective core cross-section area
• B_max = maximum flux density swing you allow

Solving for N_p:

N_p ≈ V_p,rms / (4 · f · A_e · B_max)

Key point: for a given core and frequency there is a minimum number of primary turns. If you use fewer, B gets too high, core loss explodes, and you risk saturation.

The secondary turns are set by the ratio:

N_s = n · N_p

So the chosen ratio n directly determines how many turns you end up with on the secondary. If n is small (big step-down), the secondary might only get 1–3 turns, with huge current and nasty AC loss consequences.

Core loss vs. turns and ratio

A common approximation for ferrite core loss density is:

P_core ≈ k · f^α · B^β · V_core

where k, α, and β depend on the ferrite material and temperature, and V_core is the core volume.

Using the square-wave relationship:

B ≈ V_p / (4 · f · N_p · A_e)

So core loss scales roughly like:

P_core ∝ f^α−β · V_p^β / (N_p^β · A_e^β) · V_core

That means increasing N_p (for the same V, f, and core):

• Reduces B
• Can strongly reduce core loss (β is usually ≥ 2)

But more turns also means more copper length and higher copper loss. And since N_s = n · N_p, increasing N_p increases both windings’ length.

Even if the ratio stays the same, changing N_p changes absolute turns and losses. Different ratios that force very small or very large N_s will lead to different winding layouts and AC behavior, even at similar N_p.

Copper loss and how the ratio shows up

Total copper loss is:

P_Cu = I_p,rms² R_p + I_s,rms² R_s

With DC resistance:

R = ρ · ℓ / A

where ρ is resistivity, ℓ is conductor length, and A is cross-section. For windings we can treat:

• R_p ∝ N_p / A_p
• R_s ∝ N_s / A_s

At high frequency the effective AC resistance can be much larger than the DC resistance because of skin effect and proximity effect. These get worse when:

• The conductor is thick
• The winding stack is tall or poorly interleaved
• Currents are large and magnetic fields are strong

We also know from the ideal relationships:

• N_s = n · N_p
• I_s = I_p / n

So copper loss behaves qualitatively like:

P_Cu ≈ I_p² R_p + (I_p² / n²) · R_s

where both R_p and R_s depend on N_p, n · N_p, and the chosen wire sizes.

Important consequences:

• Very small n (big step-down) means high I_s → large conductor cross-section → more AC losses and window-fill headaches.
• Very large n (big step-up) means many secondary turns → higher R_s, more leakage inductance, and more stray capacitance.

In practice there is a “sweet spot” range of turns ratios that keep both windings reasonable in terms of copper, AC loss, and layout.

Magnetizing inductance and no-load current

Magnetizing inductance for a given ferrite core is roughly:

L_m ≈ μ₀ μ_r · (N_p² · A_e / l_e)

where μ_r is relative permeability and l_e is the effective magnetic path length.

The magnetizing current is then approximately:

I_m ≈ V_p / (2π f L_m)

Because L_m ∝ N_p², fewer primary turns means:

• Smaller L_m
• Larger magnetizing current
• More idle copper and core loss at no load or light load

Again, cuts in N_p to chase a particular ratio or duty cycle can hurt efficiency even if the output voltage still looks fine.

SMPS context: ratio, duty cycle, and system efficiency

In ferrite SMPS transformers (forward, flyback, full-bridge, etc.) the transformer is part of a bigger power stage. The turns ratio affects not just the transformer but the entire converter operating point.

For a simple forward converter (idealized):

V_out ≈ D · V_in · (N_s / N_p) = D · V_in · n

where D is the duty cycle.

So for a given V_in and V_out, you can trade turns ratio against duty cycle. That gives you some important constraints:

• Too small n → very high duty cycle → more switching stress, higher magnetizing current, potential control and EMI issues.
• Too large n → very low duty cycle → high peak currents, poor utilization of core and copper, and more conduction loss in switches and rectifiers.

In a real design you balance:

• Transformer core and copper losses
• Switch and rectifier conduction + switching losses
• Leakage inductance, snubbers, and EMI

That is why different turns ratios, even on the same core, can produce significantly different overall efficiency in the finished converter.

Design takeaways for ferrite transformers

Ideal vs. real: in the ideal model, turns ratio only affects voltage and current. In the real world, it also drives:

• How many turns you must use
• How much current each winding carries
• How you can physically arrange the windings

Those three things are what set your core and copper losses.

Primary turns are fixed mainly by:

• Applied voltage
• Frequency
• Core cross-section
• Allowed B_max (core loss and saturation margin)

Secondary turns are then set by:

• The turns ratio needed for the target output voltage
• Current handling and choice of wire or copper strip
• Layout, leakage inductance, and AC loss constraints

Extreme turns ratios are usually bad news:

• Step-down with very small n → brutal secondary currents, thick copper, high AC losses
• Step-up with very large n → many turns, high resistance, big leakage, high stray capacitance

Real design is a trade-off between:

• Core loss (needs enough turns)
• Copper loss (needs not too many turns, and good conductor choice)
• Window fill and parasitics (needs sensible geometry)
• Converter duty cycle, semiconductor stresses, and EMI

If you keep those trade-offs in mind, “turns ratio” stops being a magic number and becomes what it really is: the knob that ties together voltage, current, core flux, copper geometry, and overall efficiency.

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